As part of an on-going effort to be able to support non-opengl drivers
for Cogl this splits out the opengl specific code from cogl-buffer.c
into driver/gl/cogl-buffer-gl.c
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 4d7094a979ff2cbbe4054f4a44ca05fc41a9e447)
