Separate color glyphs from monochrome glyphs in glyph cache

Color fonts may also contain perfectly normal monochrome glyphs. We currently draw these into a RGB cache and treat them as color glyphs, which is a waste of memory. As more and more fonts start adding a small set of emoji color glyphs, this will be increasingly problematic.

In the font engine, we should detect the ranges that map to color glyphs. This involves actually reading the COLR, SBIX, CBDT and SVG tables to figure it out. The glyphs mapped in these tables should be drawn into an RGB cache, but all the other glyphs should be put in an A8 cache like before.

There is probably quite a bit of replumbing needed in both Qt Quick and the raster paint engine to make this work, but it should provide good value for the effort.