Metadata: rudimentary support for XMP metadataa in MP4-based videos

XMP is a media-metadata standard based on XML which may be used
across a variety of media formats. Some video-processing software
writes XMP data without updating the native metadata fields.
Therefore, we should aim at reading XMP metadata and give priority
of XMP data over native fields.

Pros:
	- Support for *all* common media formats.
Cons:
	- XML (complex, verbose, chaotic).
	- Does not even come close to fulfilling its promise of being
	  well defined (see below).

Implement a simple XMP-parser using libxml2. Connect the XMP-parser to
the existing Quicktime/MP4 parser.

First problem encountered: According to the spec, XMP data supposed
to be put in the 'XMP_' atom. But for example exiftools instead
writes an 'uuid' atom with a special 16-byte uid. Implement both,
more options will probably follow.

Second problem: two versions of recording the creation date were found
  1) The content of a <exif:DateTimeOriginal> tag.
  2) The xmp::CreateDate attribute of a <rdf:Description> tag.

Here too, more versions are expected to surface and will have
to be supported in due course (with an obvious priority problem).

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/xmp_parser.cpp

@@ -0,0 +1,138 @@
+#include "xmp_parser.h"
+#include "subsurface-string.h"
+
+#include <libxml/parser.h>
+#include <libxml/tree.h>
+#include <cctype>
+
+extern "C" timestamp_t utc_mktime(struct tm *tm); // declared in core/dive.h
+
+static timestamp_t parse_xmp_date(const char *date)
+{
+	// Format: "yyyy-mm-dd[Thh:mm[:ss[.ms]][-05:00]]"
+	int year, month, day;
+	if (sscanf(date, "%d-%d-%d", &year, &month, &day) != 3)
+		return 0;
+
+	int hours = 0, minutes = 0, seconds = 0, milliseconds = 0;
+	int timezone = 0;
+
+	// Check for time part
+	if ((date = strchr(date, 'T')) != nullptr) {
+		++date; // Skip 'T'
+		if (sscanf(date, "%d:%d:%d.%d", &hours, &minutes, &seconds, &milliseconds) < 2)
+			return 0;
+
+		// Check for timezone part. Note that we simply ignore 'Z' as that
+		// means no time zone
+		while (*date && *date != '+' && *date != '-')
+			++date;
+		if (*date) {
+			int sign = *date == '+' ? 1 : -1;
+			int timezone_hours, timezone_minutes;
+			++date;
+			if (sscanf(date, "%d:%d", &timezone_hours, &timezone_minutes) != 2)
+				return 0;
+			timezone = sign * (timezone_hours * 60 + timezone_minutes) * 60;
+		}
+	}
+
+	// Round to seconds, since our timestamps are in seconds
+	if (milliseconds >= 500)
+		seconds += 1;
+
+	struct tm tm = { 0 };
+	tm.tm_year = year - 1900;
+	tm.tm_mon = month - 1;
+	tm.tm_mday = day;
+	tm.tm_hour = hours;
+	tm.tm_min = minutes;
+	tm.tm_sec = seconds;
+
+	timestamp_t res = utc_mktime(&tm);
+	res += timezone;
+
+	return res;
+}
+
+static timestamp_t extract_timestamp_from_attributes(const xmlNode *node)
+{
+	for (const xmlAttr *p = node->properties; p; p = p->next) {
+		const xmlChar *ns = p->ns ? p->ns->prefix : nullptr;
+
+		// Check for xmp::CreateDate property
+		if (!strcmp((const char *)ns, "xmp") && !strcmp((const char *)p->name, "CreateDate")) {
+			// We only support a single property value
+			if (!p->children || !p->children->content)
+				return 0;
+			const char *date = (const char *)p->children->content;
+			return parse_xmp_date(date);
+		}
+	}
+	return 0;
+}
+
+static timestamp_t extract_timestamp(const xmlNode *node)
+{
+	// We use a private stack, so that we can return in one go without
+	// having to unwind the call-stack. We only recurse to a fixed depth,
+	// since the data we are interested in are at a shallow depth.
+	// This can be increased on demand.
+	static const int max_recursion_depth = 16;
+	const xmlNode *stack[max_recursion_depth];
+	stack[0] = node;
+	int stack_depth = 1;
+
+	while (stack_depth > 0) {
+		const xmlNode *node = stack[stack_depth - 1];
+		// Parse attributes
+		timestamp_t timestamp = extract_timestamp_from_attributes(node);
+		if (timestamp)
+			return timestamp;
+
+		// Parse content, if not blank node. Content can only be at the second level,
+		// since it is always contained in a tag.
+		// TODO: We have to cast node to pointer to non-const, since we're supporting
+		// old libxml2 versions, where xmlIsBlankNode takes such a pointer. Remove
+		// in due course.
+		if (!xmlIsBlankNode((xmlNode *)node) && stack_depth >= 2) {
+			const xmlNode *parent = stack[stack_depth - 2];
+			// If this is a text node and the parent node is exif:DateTimeOriginal, try to parse as date
+			if (!node->ns && parent->ns &&
+			    same_string((const char *)parent->ns->prefix, "exif") &&
+			    same_string((const char *)parent->name, "DateTimeOriginal")) {
+				const char *date = (const char *)node->content;
+				timestamp_t res = parse_xmp_date(date);
+				if(res)
+					return res;
+			}
+		}
+
+		// If there are sub-items and we haven't reached recursion depth, recurse
+		if (node->children && stack_depth < max_recursion_depth) {
+			stack[stack_depth++] = node->children;
+			continue;
+		}
+
+		// Advance stack to next node in this level
+		while (stack_depth > 0) {
+			if ((stack[stack_depth - 1] = stack[stack_depth - 1]->next) != nullptr)
+				break;
+			// No more nodes at this level -> go up a level.
+			--stack_depth;
+		}
+	}
+	return 0;
+}
+
+timestamp_t parse_xmp(const char *data, size_t size)
+{
+	const char *encoding = xmlGetCharEncodingName(XML_CHAR_ENCODING_UTF8);
+	// TODO: What do we pass as URL-parameter?
+	xmlDoc *doc = xmlReadMemory(data, size, "url", encoding, 0);
+	if (!doc)
+		return 0;
+	timestamp_t res = extract_timestamp(xmlDocGetRootElement(doc));
+	xmlFreeDoc(doc);
+	return res;
+}