Implemented resolution change workaround on SDL2

SDL2 does not support changing display resolutions independently of an
SDL window. As a workaround, if the user uses ChangeResolution and then
makes a GameWindow fullscreen, we use old-style SDL fullscreen which
changes the resolution. If the user makes a GameWindow fullscreen
without calling ChangeResolution first, we use the new
fullscreen-desktop mode to match the other OpenTK backends.
Merge branch 'sdlres'