Fix stack-buffer overrun in appendVAList.

Cleaned up SkString formatting APIs for consistency and removed hard-to-
use macros. Added printVAList API for orthogonality with existing
(append|prepend)VAList methods. Optimized append/prepend methods to
use printf if appending/prepending to an empty string, a surprisingly
common case.

Change-Id: I6fbc6dc84ebb060ca452f582cb4f13a3c0f7f6ec
Bug: chromium:1092743
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297276
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
2 files changed