pdf rasterization

- add Mac PDF rasterizer
- create a mac pdf bot
- blacklist some GMs that CG takes forever to rasterize
- remove obsolete --dont_write pdf feature

This new Mac -PDF bot is now the _only_ bot running PDF code.
Obviously we can expand from here.

pdfium was looking like a pain to build and/or integrate
into our own build.  pdftocairo looked promising, and is
already installed on the bots, but it is very slow and
prone to error.  CG was the next thing I tried.

Change-Id: I82b04121f484e7dd78f60a648485a09218dd5279
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/267810
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Mike Reed <reed@google.com>
67 files changed