Remove: [Video] no longer draw in a thread

Drawing in a thread is a bit odd, and often leads to surprising
issues. For example, OpenGL would only allow it if you move the
full context to the thread. Which is not always easily done on
all OSes.
In general, the advise is to handle system events and drawing
from the main thread, and do everything else in other threads.
So, let's be more like other games.

Additionally, putting the drawing routine in a thread was only
done for a few targets.

Upcoming commit will move the GameLoop in a thread, which will
work for all targets.

src/video/sdl2_opengl_v.h

@@ -36,8 +36,6 @@ protected:
 	void Paint() override;
 	bool CreateMainWindow(uint w, uint h, uint flags) override;
 
-	void PaintThread() override {}
-
 private:
 	void  *gl_context;  ///< OpenGL context.
 	uint8 *anim_buffer; ///< Animation buffer from OpenGL back-end.