SkPDF: do all rasterScale calculations in one place.

This is just clearer to me.

Change-Id: I4fb512cf70ceac39417d8d6020e707f4436ec209
Reviewed-on: https://skia-review.googlesource.com/153663
Commit-Queue: Hal Canary <halcanary@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
5 files changed