Redesign wlr_texture

- Textures are now immutable (apart from those created from raw
  pixels), no more invalid textures
- Move all wl_drm stuff in wlr_renderer
- Most of wlr_texture fields are now private
- Remove some duplicated DMA-BUF code in the DRM backend
- Add more assertions
- Stride is now always given as bytes rather than pixels
- Drop wl_shm functions

Fun fact: this patch has been written 10,000 meters up in the air.

include/backend/drm/drm.h

@@ -27,7 +27,6 @@ struct wlr_drm_plane {
 
 	// Only used by cursor
 	float matrix[9];
-	struct wlr_texture *wlr_tex;
 	struct gbm_bo *cursor_bo;
 	bool cursor_enabled;
 	int32_t cursor_hotspot_x, cursor_hotspot_y;