As part of an on-going effort to enable non-opengl drivers for Cogl this
splits out the opengl specific code in cogl-clip-stack.c into
cogl/driver/cogl-clip-stack-gl.c
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit acf989f1bb628282c53d1249b2e3fc6f6579f1e9)
