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.
Instead of checking whether the wlr_egl dependencies are available
in the GLES2 code, introduce internal_features['egl'] and check
that field.
When updating the EGL dependency list, we no longer need to update
the GLES2 logic.
