gsk: Avoid downloading GL textures when possible

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.