This adds a test to make sure that if the same pipeline is used to draw
two primitives, one which doesn't need blending because it has an opaque
color associated, and another using a color attribute that requires
blending then Cogl should recognize that it needs to enable blending for
the second primitive even though the pipeline hasn't changed.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit c15d91f1c6293bebd4494d1e20586121483cddef)
