Sk4px: Difference and Exclusion

This will cause minor (off-by-one) diffs due to a little lost precision:
	colortype_xfermodes
	mixed_xfermodes
	xfermodes2
	xfermodeimagefilter
	xfermodes3
	xfermodes

Desktop:
Xfermode_Difference_aa	9.77ms -> 7.32ms	0.75x
 Xfermode_Exclusion_aa	8.49ms -> 6.21ms	0.73x
   Xfermode_Difference	  17ms -> 7.54ms	0.44x
    Xfermode_Exclusion	13.5ms -> 5.09ms	0.38x

N7:
Xfermode_Difference_aa	32.2ms -> 27.6ms	0.86x
   Xfermode_Difference	43.9ms ->   32ms	0.73x
 Xfermode_Exclusion_aa	40.5ms -> 26.7ms	0.66x
    Xfermode_Exclusion	71.5ms -> 23.9ms	0.33x

This wraps up the xfermodes implemented in Sk4f.

BUG=skia:

Review URL: https://codereview.chromium.org/1141213002
5 files changed