Rendering refactor:
* Move API related code to SDLRenderingDevice
* Wrap SDL_Surface and positioning into Sprite class
* Move all rendering to rendering device
Rename Sprite::map_pos to Sprite::dest
Remove unused prio
Merge remote-tracking branch 'remotes/upstream/master' into rendering
Define Image class
Move remaining map rendering to context
FontEngine refactoring
Make screen private member of SDLRenderDevice
Make Sprite::dest private
Wrap SDL_FillRect() into fillImageWithColor()
Try to fix clang build
Move all essential function prototypes to RenderDevice.h
Wrap SDL_SetAlpha and SDL_SetColorKey
Turn Image #define into an Image class
Move more code to RenderDevice
Add setGamma and listModes to RenderDevice

setColorKey and setAlpha are now part of RenderDevice as well.
Move more functions to RenderDevice class
Merge pull request #7 from dorkster/render_refactor2

Moving more functions to the RenderDevice class
Merge pull request #911 from igorko/rendering

Rendering refactor