Fix vertex indexing error when using ImGui skiaWidgets

skiaWidget adds fake "images" to the ImGui draw list. Those have
vertices allocated for them (even though we don't consume them). We
still need to skip over those if any ImGui widget rendering happens
after the callback, though.

Bug: skia:
Change-Id: Ia0243e600bd32d90ba97ac9fbe58b715f70aa83a
Reviewed-on: https://skia-review.googlesource.com/c/190664
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed