compositor/main.c: use pixman renderer by default when gl-renderer not enabled
When the gl-renderer is not enabled, weston fails to start, as it doesn't automatically fallback to the pixman renderer, which is always enabled.
This commit changes the drm-backend to set by default the --use-pixman option to true when the gl-renderer is disabled (BUILD_DRM_GBM is not defined).
Signed-off-by: Thomas Petazzoni thomas.petazzoni@bootlin.com