Allow drawing the selected text background earlier

[martincapello]

This PR modifies the order in which some things are done when each run of text is processed when the shaping function is executed:

• Now, the selected text background drawing is delegated before drawing any character. In this way if the glyphs in a run overlap with each other the background drawing of the current glyph won't overwrite the previous glyph.
• Now, if there is a background color set, it is painted before drawing any character and before delegating the selected text background painting.

Both of the above things are possible because now we first calculate the union of all glyphs bounds.

[dacap]

I didn't test this yet, so here are some minor changes.

The DrawTextDelegate is deprecated and we'd like to remove it in the future, but it's still used in ui::Entry so it's fine to fix bugs on it 👍

[dacap]

A minor change, I've tested on Linux and macOS with "Apple Chancery" and it works great 👍

[dacap]

We can prealloc the expected size of glyphsBounds:

Suggested change

	std::vector<gfx::RectF> glyphsBounds;
	for (int i = 0; i < info.glyphCount; ++i) {
	auto bounds = info.getGlyphBounds(i);
	glyphsBounds.push_back(bounds);
	std::vector<gfx::RectF> glyphsBounds(info.glyphCount);
	for (int i = 0; i < info.glyphCount; ++i) {
	auto bounds = info.getGlyphBounds(i);
	glyphsBounds[i] = bounds;