compositor-drm: Disable planes when zoomed or capturing

When the entire output is transformed or we're capturing the output
for screenshot or video, disable all output specific overlays
(drm planes, hw cursors etc) and move all surfaces into the primary
plane.
diff --git a/src/compositor.h b/src/compositor.h
index 5cc45da..ea6d82f 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -165,6 +165,7 @@
 	int dirty;
 	struct wl_signal frame_signal;
 	uint32_t frame_time;
+	int disable_planes;
 
 	char *make, *model;
 	uint32_t subpixel;