Instead of having 3 different fragment shaders for textures and 1
more for quads, unify them all and compile different variants via
a SOURCE constant.
In the future, we will have more variant dimensions: pre-multiplied
alpha on/off, color transformation, etc. It will become inpractical
to have one file per combination.
Weston has a similar approach:
https://gitlab.freedesktop.org/wayland/weston/-/blob/main/libweston/renderer-gl/fragment.glsl
Instead of having a C file with strings for each shader, move each
shader into its own file. Use a small POSIX shell script to convert
the files into C strings (can't wait for C23 #embed...).
The benefits from this are:
- Improved readability and syntax highlighting.
- Line numbers in shader compiler errors are easier to make sense of.
- Consistency with the Vulkan renderer.
- Shaders will become more complicated as we add color management
features.
