update placeholder approx op count

Now that approxOpCount() can recurse into sub pictures,
adding ~0 will overflow its total count.

New tests fail at head, pass now.

Change-Id: Icdb7d8a062739724207c9a6369734510f00f2ffd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/311774
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed