This function sets the master clipping against which all local clip regions are clipped. Calling this will also reset the local clipping region.
int x1 Horizontal location of the upper left corner of the global clipping region.
int y1 Vertical location of the upper left corner of the global clipping region.
int x2 Horizontal location of the lower right corner of the global clipping region.
int y2 Vertical location of the lower right corner of the global clipping region.