Fix overflow in FT bounding boxes

There was an overflow with LONG_MIN being negated if it snuck
into the font's bounding box. We now check that in two places
to avoid the issue.

I also was a bit confused by the ~63 (which was doing subpixel
alignment in Fdot6), so I used some self-documenting function names.

Oh, I think the use of the portable fontmgr in FuzzMain was
a mistake as it prevented the actual freetype fontmgr from
being used during reproduction (which is what had this bug).

Bug: oss-fuzz:484857317
Change-Id: I484c107af340d40c6e08c16f2bd5827747042ac8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1220856
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>