glsl: include variable name in error messages about initializers
glsl: include variable name in error messages about initializers

Also fix style / wrong indentation along the way and make the messages
more uniform.

Reviewed-by: Samuel Iglesias Gonsálvez <b753b212f30aed827c3eea2e437609dc37310153@igalia.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler