Remove fmt parameter from wlr_texture_write_pixels

This uses dummy format values (0xFFFFFFFF) for immutable wlr_gles2_texture objects since we don't always know their formats (and don't write the textures anyways so why bother with additional conversions in that case). The buffer damage handling had to be changed a bit; the previous implementation seemed to rely on being able to change the textures format (which was broken for the gles2 renderer since has_alpha was not updated on format change). Breaking change, i guess this is relevant for sway and @Ongy @Timidger @ammen99 (add anyone i missed) if it gets merged.