gsk: Avoid copying GL texture in more cases

gsk: Avoid copying GL texture in more cases

When the GL texture already has a mipmap, we don't
have to download and reupload it to generate one.

We differentiate the handling for texture scale nodes,
where we do want to force the mipmap creation even if
it requires us to reupload the GL texture, and plain
texture nodes, where we just take advantage of a
preexisting mipmap to allow trilinear filtering for
downscaling, or create one if we have to upload the
texture anyway.