This adds a CoglObject called CoglSnippet which will be used to store
strings used as GLSL snippets to be attached at hook points to a
CoglPipeline. The snippets can currently contain three strings:
declarations - This will be placed in the global scope and is intended
to be used to declare uniforms, attributes and
functions.
pre - This will be inserted before the hook point.
post - This will be inserted after the hook point.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
