Fixed issue where large blur radii

would blur less than smaller blur radii

Changed SkImageFilter blur creation to include
SkTileMode::kClamp parameter to use edge pixels
for blur kernels that are larger than the source

Fixes: 167493483
Test: Added test to CTS ShaderTest
Change-Id: Ibbe1e0dfaf46413fcae665620fd154b9bf9897c9