wlr_scene: Fix scaling with default source_box

Default to texture instead of destination_box geometry.

Example before and after the change using the gles2 renderer:

All boxes share the same wlr_buffer and smaller and bigger boxes were scaled using wlr_scene_buffer_set_dest_size().
The 3 boxes on the bottom were additionally transformed via wlr_scene_buffer_set_transform(scene_buffer, WL_OUTPUT_TRANSFORM_90)