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.

backend/wayland/output.c

@@ -99,8 +99,6 @@ static bool wlr_wl_output_set_cursor(struct wlr_output *_output,
 		return true;
 	}
 
-	stride *= 4; // stride is given in pixels, we need it in bytes
-
 	if (!backend->shm || !backend->pointer) {
 		wlr_log(L_INFO, "cannot set cursor, no shm or pointer");
 		return false;