Rewrite SkVertices serialization to use SkReadBuffer/SkWriteBuffer

These classes are much safer (there's no way to safely deserialize a
string with SkReader32 without knowledge of how it works internally).
Prior to this CL, SkVertices was the only complex type that had manual
serialization using the lower level types - now it works like everything
else. Additionally: the versioning can now be tied to picture versions
going forward (like everything else).

Bug: oss-fuzz:22909
Bug: oss-fuzz:22918
Bug: skia:9984
Bug: skia:10304
Change-Id: I3cf537eb765b5c8ce98b554c0f200e5d67c33d14
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293349
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed