viewer: Add a "Path renderer" dropdown menu

BUG=skia:

Change-Id: Ia3ed812d24f0f83631ab238bc418a3c95d49b9dc
Reviewed-on: https://skia-review.googlesource.com/9000
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
6 files changed