Instead of having to extend cogl_is_buffer with new buffer types
manually this now adds a new COGL_BUFFER_DEFINE macro to be used instead
of COGL_OBJECT_DEFINE for CoglBuffer subclasses. This macro will
automatically register the new type with ctx->buffer_types which will
iterated by cogl_is_buffer. This is the same coding pattern used for
CoglTexture.
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