Supported when ARB_framebuffer_no_attachments is available.

It was reported that no attachments framebuffers trigger GPU hangs
in The Witcher 3 on some system configurations with radeonsi driver.
MESA_EXTENSION_OVERRIDE=-GL_ARB_framebuffer_no_attachments can be used
as a workaround until it's fixed.
Signed-off-by: Józef Kucia <jkucia@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>