docs: Document VC4_DEBUG envvar
docs: Document VC4_DEBUG envvar

Signed-off-by: Rhys Kidd <0b3b315d08353e2046d43a86d41e3fd1dff3d6ad@gmail.com>
Signed-off-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler