Initial SDL2 implementation
Merge branch 'SDL2port' of https://github.com/igorko/flare-engine into sdl2
Implement more SDL2 rendering
Merge branch 'SDL2port' of https://github.com/igorko/flare-engine into sdl2
Add SDL2 include/library directories to CMakeLists.txt

Use '-DUSE_SDL2=On' when running cmake
Fix loading cutscene images
Fix SDL2 text rendering
Fix switching resolution in-game with SDL2
Fix loading magic pink images
Implement pixel/line/rect drawing in SDL2 Renderer
Don't create new surface/texture when copying Sprite
Remove setAlpha() and setColorKey()

These functions were being used to no effect. We set the alpha for blank
surface using fillImageWithColor(). The only place we used setColorKey()
was on a surface created with createSurface(), which sets the color key
by itself.
Fix texture stretching with negative clip rectangles
Implement setGamma() for SDL2
Implement scaleSurface() for SDL2
Merge remote-tracking branch 'remotes/upstream/master' into SDL2port
Merge remote-tracking branch 'remotes/upstream/master' into SDL2port
Merge branch 'SDL2port' of https://github.com/eezstreet/flare-engine