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