While I'm here, make pixelsPerPt local on android and darwin/amd64.
The one place where it's global is darwin/arm, which has another
similar global (screenScale).
Change-Id: I5897e7e5341afca1976fdf0215fb4f6fe2f411be
Reviewed-on: https://go-review.googlesource.com/13446
Reviewed-by: Nigel Tao <nigeltao@golang.org>