Make SkBlurImageFilter capable of cropping during blur (GPU path).
This is the GPU equivalent of https://codereview.chromium.org/
1415653003/.
It requires passing down the bounds of the crop rect (srcBounds), and
turning the blur 3-patch optimization in convolve_gaussian() into a 5-patch:
clear above and below srcBounds, blur with bounds checks inside left and
right rects, blur without bounds checks in middle rect.
Note: this change causes minor pixels diffs in the
imagefilterscropexpand GM: for odd crop positions relative to the
dstBounds, we are now correctly resampling at an even pixel boundary.
BUG=skia:4502, skia:4526
Review URL: https://codereview.chromium.org/
1431593002