fix blank catalog images

Some catalog images are blank because
the image wasn't selected in the fiddle
example.

Also continue to add documentation and
examples for SkRRect

R=caryclark@google.com
NOTRY=true

Docs-Preview: https://skia.org/?cl=138382
Bug: skia:6898
Change-Id: I282a14ffcbaf2f8383294724900960f5ba40a2aa
Reviewed-on: https://skia-review.googlesource.com/138382
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
9 files changed