USBTablet "deadzone" in full screen

[kev.mitch]

When running a OSX Mountain Lion (<HID Pointing="USBTablet" Keyboard="USBKeyboard"/>), there is a deadzone at the edge of the guest screen. In other words, there is a region where physically moving the mouse resuts in no cursor movement in either the guest or host. This is arguably a feature when the guest is running in a window. However, when it is running in full screen, it leads to the mouse getting "stuck" at the edge of the screen.

[aeichner]

Please reopen if still relevant with a recent VirtualBox release.