Remove unnecessary bound-box parameter from clipping functions

The bounding rectangle actually comes from the polygon. Therefore, it's
not needed. Removed from the following functions in OutputDevice, et al

+ ClipAndDrawGradient
+ XORClipAndDrawGradient
+ ClipAndDrawGradientMetafile

Change-Id: I4a87edcddb8895871982f0448854e1c0854124bc