gdk: Don’t convert textures into cairo surface on every draw
When using the cairo renderer, textures currently get converted to BGRA8888 with premultiplied alpha on every gsk texture draw. This commit keeps a reference to the initial surface (or the first one getting created) to avoid further conversions.
In addition to !4481 (merged), this makes scrolling through many PNGs actually fast on a Cortex-A7.