meson,mesa: complete shader replacement infrastructure
for custom shader replacements enabled by -Dcustom-shader-replacement=path.
process_shaders.py should generate shader_replacement.h, which should contain shaders and their substitutions.
for custom shader replacements enabled by -Dcustom-shader-replacement=path.
process_shaders.py should generate shader_replacement.h, which should contain shaders and their substitutions.