fix scan converter arena alloc

Removing the 16K alloc sped up pathops_unittest -x -V
Release on Windows 7; time went from 14 minutes to 4.

R=herb@google.com
BUG=skia:6509

Change-Id: If43c9ad6745961e0079a7f4f6560c6fa2a7847ef
Reviewed-on: https://skia-review.googlesource.com/13507
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Herb Derby <herb@google.com>
2 files changed