profile: reimplement DivePercentageItem

The tissue percentages were realized as 16 independent polygons.
That didn't work at all with the new absolute scaling.

Reimplement the item and blast it onto a pixmap. Not only is
this artifact-free, it also should (hopefully) be quite a bit
more efficient than painting numerous lines.

In contrast to the old code, this does access the plot_info
structure directly instead of using the model. Not so much
for performance reason, but rather to make things more robust:
We have a strongly typed language. Why would we shoehorn data
through the weakly typed QVariant and mess with wierd
index-arithmetics. Makes no sense to me. Qt-model have to
be used for interfacing with Qt. They are terrible for
intra-application data transfer.

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

profile-widget/divecartesianaxis.cpp

@@ -359,6 +359,12 @@ double DiveCartesianAxis::minimum() const
 	return min;
 }
 
+std::pair<double, double> DiveCartesianAxis::screenMinMax() const
+{
+	return position == Position::Bottom ? std::make_pair(rect.left(), rect.right())
+					    : std::make_pair(rect.top(), rect.bottom());
+}
+
 void DiveCartesianAxis::setColor(const QColor &color)
 {
 	QPen defaultPen = gridPen();