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.

examples/multi-pointer.c

@@ -149,7 +149,7 @@ static void handle_input_add(struct compositor_state *state,
 		sample->compositor);
 
 	struct wlr_xcursor_image *image = sample->xcursor->images[0];
-	wlr_cursor_set_image(cursor->cursor, image->buffer, image->width,
+	wlr_cursor_set_image(cursor->cursor, image->buffer, image->width * 4,
 		image->width, image->height, image->hotspot_x, image->hotspot_y, 0);
 
 	wl_list_insert(&sample->cursors, &cursor->link);