Added tests to verify BlurShader edge treatments

Added tests to verify the default edge treatment to
use transparent pixels for blur kernels that extend
beyond the source as well as to use edge pixels.

Blurring with a large radius and sampling transparent
pixels should generate a result that is mostly transparent.
However, blurring with a large radius and sampling the edge
pixels of the source should yield a result that matches
the source content more closely.

Fixes: 167714368
Test: this
Change-Id: Ib67472de0fde4eece0d236c53e67ddb7beb490e9