Fixes, Cleanups, Removes GL.
Convert Widget::render interface from (int2 position, int2 size) to (const Image& target).

Advantages:
- Removes external globals: Image framebuffer, array<Rect> clipStack, Rect currentClip.
- Removes explicit position offset in graphics primitives drawing calls.
- Simpler target settings.

Drawback:
- ScrollArea becomes indirect (when in use).
TODO: Keep ScrollArea buffer between render for fast scroll (needs to be signaled whether underlying viewed Widget is uptodate).
Fix oxygen compatible window background.
Fix sRGB blending.
Fix ScrollArea.