Update dftext expectations after https://codereview.chromium.org/772633003/.

BUG=skia:3195, skia:3188
TBR=scroggo@google.com

Review URL: https://codereview.chromium.org/778043003
Constify CreatePictureShader's picture arg.

R=robertphillips@google.com

Review URL: https://codereview.chromium.org/768113004
Add new GM (filterfastbounds)

This new GM visualizes the fast bounds computed by various image-filter-based SkPaints. This is lead up to fixing some issues in fast bound computation.

BUG=418417

Review URL: https://codereview.chromium.org/788613003