introduce virgl_context as the base class of vrend_decode_ctx
This MR introduces virgl_context as the base class for vrend_decode_ctx, and update virglrenderer.c to be more generic. This allows us to introduce another context type (e.g., vkrend_context inheriting from virgl_context) in the future.
The resource paths are still vrend-specific. !364 (merged) is an RFC to deal with resources.