Embedder API should warn when software and GL renderer backing stores are mixed.

[chinmaygarde]

Currently, it is embedder responsibility to be consistent with the client rendering API of the backing stores being used with the custom compositor. However, mixing them will lead to significant performance issues (if it works at all in the case of a software root surface renderer config and a an OpenGL compositor backing store).

The embedder API must make sure the client rendering API of the compositor backing stores matches the root renderer config and return an error otherwise.