wip: color profile support, linear compositing

This branch collects work from last year on color profile support and linear compositing.

Open issues for this work:

• Is linear compositing what we want ? (we kinda need it for better fonts)
• Send colorprofile information to the compositor instead of post-processing the framebuffer back into non-linear, premultiplied sRGB
• Do we want to support colorspaces like HLS or HWB ? (question is how such data would reach GDK. At last as far as individual colors are concerned, GdkRGBA always delivers RGB, and we do handle the conversion on the GTK side. It might come up if we wanted to support CSS syntax for doing gradients in other colorspaces)
• Track down all the test failures
• Make the new debug flags always available
• Implement transforms for HDR cicp triples (lcms can't do those)
• Do colorspace conversion in a shader for GL
• Rename to the latest agreed on naming: colorspace -> colorstate
• Make the heif demos build conditionally