Add a GaussianBlurFilter to SkiaRE using Skia blur

This is using Skia's native blur algorithm which is a fully-correct blur
using separate X/Y convolution passes in a pre-baked shader.

The performance is significantly worse than Kawase or dual-filtered
Kawase but it sets the high bar for accuracy and the low bar for
performance.

This can be switched with KawaseBlurFilter by updating the source in
SkiaGLRenderEngine to instantiate GaussianBlurFilter instead.

Test: atest BlurTests
Bug: 185365391
Change-Id: I29fba84e408c75bc8ef7011b6e40f45cb78eabab