Add Surface class stub
Document Surface class
Implement surface locking
Fix copypasta
Add Point/Rect constructors taking SDL_Point/SDL_Rect
Implement most Surface functions
Add SDL_image surface loaders
Allow to create Texture from Surface
Use surface in example
Merge branch 'surface'

Conflicts:
	CMakeLists.txt
Add constructors from existing SDL2 objects
Only add image-related sources to the project if SDL_image support is enabled
Include SDLImage conditionally