* Tablet Pressure support in GHOST

This is 'ported' from Nicholas Bishop's sculpting GSoC tree. I'm bringing it
over now so a) it can be there for when lukep does his GHOST refactor b) it's
something that GHOST should have anyway, particularly now there's interest in
painting tools and c) it's missing support in Windows, so hopefully now some
enterprising Windows coder can add that more easily in the main bf tree.

Right now X11 and Mac OS X are supported. I added and can maintain the Mac OS X
part, but I'm not familiar with the Xinput stuff, which Nicholas wrote. Both
X11 and Mac are collecting active device and pressure, and Mac is also
collecting x and y tilt data. Up to coders how they want to use this info! :)

Although the data's coming in, I haven't actually made this do anything. I
thought it best to leave it to brecht to figure out what he wants to do with the
painting stuff, and I wonder what other interesting uses there could be for it
(proportional edit?). I'll write implementation details in a separate mail to
the committers list.

intern/ghost/GHOST_IWindow.h

@@ -201,6 +201,12 @@ public:
 	 */
 	virtual void setUserData(const GHOST_TUserDataPtr userData) = 0;
 	
+	/**
+	 * Returns the tablet data (pressure etc).
+	 * @return The tablet data (pressure etc).
+	 */
+	virtual const GHOST_TabletData* GetTabletData() = 0;
+	
 	/***************************************************************************************
 	 ** Cursor management functionality
 	 ***************************************************************************************/