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

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