output: take a wlr_buffer in set_cursor

Instead of passing a wlr_texture to the backend, directly pass a
wlr_buffer. Use get_cursor_size and get_cursor_formats to create
a wlr_buffer that can be used as a cursor.

We don't want to pass a wlr_texture because we want to remove as
many rendering bits from the backend as possible.

include/backend/wayland.h

@@ -83,9 +83,7 @@ struct wlr_wl_output {
 	struct {
 		struct wlr_wl_pointer *pointer;
 		struct wl_surface *surface;
-		struct wlr_swapchain *swapchain;
 		int32_t hotspot_x, hotspot_y;
-		int32_t width, height;
 	} cursor;
 };