Refactor position computation to enable device space "nudge"

To match raster's handling of BW geometry we want to be able to perform a device space "nudge" on all geometry. This CL sets us up to be able to do that in GrGLVertexBuilder::transformToNormalizedDeviceSpace.

BUG=423834
TBR=bsalomon@google.com

Review URL: https://codereview.chromium.org/854013002
15 files changed