Allow separate X/Y values for gaussian blur std-deviation.

The Gaussian blur canvas layer filter previously only supported one
value for the `stdDeviation` attribute. However, the SVG specification
supports anisotropic blurs, with different values for the X and the Y
axis:

https://drafts.fxtf.org/filter-effects/#element-attrdef-fegaussianblur-stddeviation

Canvas filters are defined as following the SVG specification, so
we must support distinct X and Y blur std-deviations.

Bug: 1451837
Low-Coverage-Reason: TESTS_IN_SEPARATE_CL
Change-Id: I5be742a092cde5738e1ade3117c05ccabebd9481
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5067734
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1235786}