mesa/st: Create front renderbuffer on the fly when supplied with a surface

mesa/st: Create front renderbuffer on the fly when supplied with a surface

Normally, the mesa/st would create a fake front buffer out of a
client-allocated surface.

In the DRI setting, however, st/dri provides a front buffer surface which is
created and maintained by the X server. Prefer to use this surface instead,
so that front buffer rendering and reading works correctly.

Signed-off-by: Nicolai Hähnle <nhaehnle@gmail.com>