Added overload to SDL_RenderCopy method to allow SourceRectangle to be passed as an IntPtr. This allows developers to pass IntPtr.Zero to take advantage of internal SDL default values for null rectangles.
Added all source/destination null parameter overload variations for RenderCopy. Added function comments to explain their use. Fixed inconsistent tabbing.
Merge pull request #33 from babelshift/RenderCopyOverload

 Issue #32: Added overload for SDL_RenderCopy method