mesa: pass context parameter to gl_renderbuffer::Delete()

mesa: pass context parameter to gl_renderbuffer::Delete()

We sometimes need a rendering context when deleting renderbuffers.
Pass it explicitly instead of trying to grab a current context
(which might be NULL).  The next patch will make use of this.

Note: this is a candidate for the stable branches.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>