Switch to fullscreen on the monitor PPSSPP is.

When using more than one monitor, PPSSPP was always switching to full screen on the main monitor.
Now it will switch to full screen on the monitor he currently is.

Many thanks to @unknownbrackets for explaining me how to do this !
Merge pull request #6551 from vnctdj/patch-1

Switch to fullscreen on the monitor PPSSPP is.