output/cursor: fix scale and transform

Many issues here:

- wlr_output_cursor_set_buffer() takes a buffer already scaled for
  the output, not a buffer with scale=1.
- wlr_output_cursor.{width,height,hotspot_x,hotspot_y} are in output
  buffer coordinates.
- render_cursor_buffer() had hardcoded no-ops for scale and
  transform, instead of using the cursor surface's.

Fixes: b64e7e88bf ("output: add output_cursor_set_texture()")

include/wlr/types/wlr_output.h

@@ -39,6 +39,8 @@ struct wlr_output_cursor {
 	bool enabled;
 	bool visible;
 	uint32_t width, height;
+	float scale;
+	enum wl_output_transform transform;
 	int32_t hotspot_x, hotspot_y;
 	struct wlr_texture *texture;
 	bool own_texture;