Fix gaussian blur for small sigma.

Fix gaussian blur for small sigma.

I broke this in https://skia.googlesource.com/skia/+/c57e0ded7d535523cfc6bf07c78e5f3479bb8c42.
That change added support for cropping during blur, but
did not do the correct thing for the small-sigma 2D matrix convolution
optimization when blurring from small-to-large textures.

The fix is pass the correct dstRect and srcOffset to convolve_gaussian_2d.
I also changed convolve_gaussian_1d() and convolve_gaussian_2d() to take
the original (non-negated) srcOffset, and to negate them when constructing
the local matrix.

Test: added a new (blur 0.3 0.3) column to imagefilterscropexpand, so that
GM will have to be rebaselined.

BUG=skia:4719,569883
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Review URL: https://codereview.chromium.org/1529313003