compositor-wayland: Add pixman/SHM fallback mode

This also adds a --use-pixman command-line option to enable the pixman
renderer explicitly.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/compositor.c b/src/compositor.c
index 31c01c5..edb13a2 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -3611,6 +3611,7 @@
 		"Options for wayland-backend.so:\n\n"
 		"  --width=WIDTH\t\tWidth of Wayland surface\n"
 		"  --height=HEIGHT\tHeight of Wayland surface\n"
+		"  --use-pixman\t\tUse the pixman (CPU) renderer\n"
 		"  --display=DISPLAY\tWayland display to connect to\n\n");
 
 #if defined(BUILD_RPI_COMPOSITOR) && defined(HAVE_BCM_HOST)