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>

src/mesa/drivers/x11/xm_buffer.c

@@ -235,7 +235,7 @@ alloc_back_buffer(XMesaBuffer b, GLuint width, GLuint height)
 
 
 static void
-xmesa_delete_renderbuffer(struct gl_renderbuffer *rb)
+xmesa_delete_renderbuffer(struct gl_context *ctx, struct gl_renderbuffer *rb)
 {
    /* XXX Note: the ximage or Pixmap attached to this renderbuffer
     * should probably get freed here, but that's currently done in