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

@@ -77,6 +77,31 @@ extern DECLSPEC SDL_Window * SDLCALL SDL_GetMouseFocus(void);
  */
 extern DECLSPEC Uint32 SDLCALL SDL_GetMouseState(int *x, int *y);
 
+/**
+ *  \brief Get the current state of the mouse, in relation to the desktop
+ *
+ *  This works just like SDL_GetMouseState(), but the coordinates will be
+ *  reported relative to the top-left of the desktop. This can be useful if
+ *  you need to track the mouse outside of a specific window and
+ *  SDL_CaptureMouse() doesn't fit your needs. For example, it could be
+ *  useful if you need to track the mouse while dragging a window, where
+ *  coordinates relative to a window might not be in sync at all times.
+ *
+ *  \note SDL_GetMouseState() returns the mouse position as SDL understands
+ *        it from the last pump of the event queue. This function, however,
+ *        queries the OS for the current mouse position, and as such, might
+ *        be a slightly less efficient function. Unless you know what you're
+ *        doing and have a good reason to use this function, you probably want
+ *        SDL_GetMouseState() instead.
+ *
+ *  \param x Returns the current X coord, relative to the desktop. Can be NULL.
+ *  \param y Returns the current Y coord, relative to the desktop. Can be NULL.
+ *  \return The current button state as a bitmask, which can be tested using the SDL_BUTTON(X) macros.
+ *
+ *  \sa SDL_GetMouseState
+ */
+extern DECLSPEC Uint32 SDLCALL SDL_GetGlobalMouseState(int *x, int *y);
+
 /**
  *  \brief Retrieve the relative state of the mouse.
  *
@@ -126,6 +151,37 @@ extern DECLSPEC void SDLCALL SDL_WarpMouseGlobal(int x, int y);
  */
 extern DECLSPEC int SDLCALL SDL_SetRelativeMouseMode(SDL_bool enabled);
 
+/**
+ *  \brief Capture the mouse, to track input outside an SDL window.
+ *
+ *  \param enabled Whether or not to enable capturing
+ *
+ *  Capturing enables your app to obtain mouse events globally, instead of
+ *  just within your window. Not all video targets support this function.
+ *  When capturing is enabled, the current window will get all mouse events,
+ *  but unlike relative mode, no change is made to the cursor and it is
+ *  not restrained to your window.
+ *
+ *  This function may also deny mouse input to other windows--both those in
+ *  your application and others on the system--so you should use this
+ *  function sparingly, and in small bursts. For example, you might want to
+ *  track the mouse while the user is dragging something, until the user
+ *  releases a mouse button. It is not recommended that you capture the mouse
+ *  for long periods of time, such as the entire time your app is running.
+ *
+ *  While captured, mouse events still report coordinates relative to the
+ *  current (foreground) window, but those coordinates may be outside the
+ *  bounds of the window (including negative values). Capturing is only
+ *  allowed for the foreground window. If the window loses focus while
+ *  capturing, the capture will be disabled automatically.
+ *
+ *  While capturing is enabled, the current window will have the
+ *  SDL_WINDOW_MOUSE_CAPTURE flag set.
+ *
+ *  \return 0 on success, or -1 if not supported.
+ */
+extern DECLSPEC int SDLCALL SDL_CaptureMouse(SDL_bool enabled);
+
 /**
  *  \brief Query whether relative mouse mode is enabled.
  *