Merged Ryan's SDL-gui-backend branch.

Adds three APIs, and implements them on X11, Cocoa, and Windows:

- SDL_CaptureMouse()
- SDL_GetGlobalMouseState()
- SDL_SetWindowHitTest()

include/SDL_mouse.h

@@ -123,6 +123,16 @@ extern DECLSPEC Uint32 SDLCALL SDL_GetRelativeMouseState(int *x, int *y);
 extern DECLSPEC void SDLCALL SDL_WarpMouseInWindow(SDL_Window * window,
                                                    int x, int y);
 
+/**
+ *  \brief Moves the mouse to the given position in global screen space.
+ *
+ *  \param x The x coordinate
+ *  \param y The y coordinate
+ *
+ *  \note This function generates a mouse motion event
+ */
+extern DECLSPEC void SDLCALL SDL_WarpMouseGlobal(int x, int y);
+
 /**
  *  \brief Set relative mouse mode.
  *