This adds a way to mark that a primitive is in use so that modifications
will generate a warning. The plan is to use this mechanism when batching
primitives in the journal to warn users that mid-scene modifications of
primitives is not allowed.
A CoglPrimitive is a retainable object for drawing a single primitive,
such as a triangle strip, fan or list.
CoglPrimitives build on CoglVertexAttributes and CoglIndices which
themselves build on CoglVertexArrays and CoglIndexArrays respectively.
A CoglPrimitive encapsulates enough information such that it can be
retained in a queue (e.g. the Cogl Journal, or renderlists in the
future) and drawn at some later time.
