GL: Crossfade nodes are scaled up in HiDPI setups
The GL Renderer should render offscreen nodes at twice their size in this case, and then not scale up the result. The current code renders the child nodes unscaled and then scales the resulting texture up.
This is mostly not noticeable since we try hard to not every draw anything to a texture, but we force offscreen drawing for crossfade nodes, and it shows in e.g. the checkmarks of checkbuttons.