Drop GBM_BO_USE_SCANOUT from the GBM_BO_IMPORT_FD import, add
GBM_BO_USE_RENDERING to the GBM_BO_IMPORT_FD_MODIFIER import.

If the DMA-BUF cannot be scanned out, gbm_bo_import with
GBM_BO_USE_SCANOUT will fail. However Xwayland doesn't need to scan-out
the buffer and can work fine without scanout. Glamor only needs
GBM_BO_USE_RENDERING.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>