gl: Fixed clipped offscreen rendering

gl: Fixed clipped offscreen rendering

When we are rendering a texture node to an offscreen,
and we have a clip, we must force the offscreen rendering.
Otherwise, the code will notice: Hey, it already is a texture
node, so no need to render it to a texture again. But when
clipping is involved, that is exactly what we want to do.

Testcase included.

Fixes: #3651