gsk: Avoid downloading GL textures when possible

I found that the gears demo was spending 40% cpu downloading a GL texture every frame, only to upload it again to another context.

While the GSK rendering and the GtkGLArea use different GL contexts, they are (usually) connected by sharing data with the same global context, so we can just use the texture without the download/upload dance. This brings gears down to < 10% cpu.