gl: Respect clip wehn drawing scale nodes

Use the same approach and only create an offscreen
that is big enough for the clipped part of the scaled
texture.

If the clipped part is still too large for a single
texture, we give up and just render the texture without
filters (using the regular texture rendering code path
which supports slicing).

The following commit will add the texture-scale-magnify-10000x
test which fails without this fix.