gm: add a flag to force perspective usage in all gms

BUG=
R=reed@google.com, epoger@google.com, borenet@google.com

Author: edisonn@google.com

Review URL: https://chromiumcodereview.appspot.com/23587029

git-svn-id: http://skia.googlecode.com/svn/trunk@11487 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed