As part of an on-going effort to enable non-opengl drivers for Cogl this
splits out the GL specific code in cogl-attribute.c into
cogl/driver/gl/cogl-attribute-gl.c
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 7e20c39c47fa176aa5062867ff273bc2c41a2f22)
