mesa: rename GroupStackDepth to CurrentGroup
mesa: rename GroupStackDepth to CurrentGroup

The variable is used as the actual index, rather than the size of the
group stack - rename it to reflect that.

Suggested-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Timothy Arceri <5dd83789ef3714029244367ba1ca09f1d7627408@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler