Have multiple plotmgrs, one for each mask format.

Have multiple plotmgrs, one for each mask format.
Only flush/purge those strikes that match our format.

R=bsalomon@google.com

Committed: https://code.google.com/p/skia/source/detail?r=11303

[jvanverth1, 2013-09-11 18:50:31]

This causes no change in GMs, and lowers the number of uploads (when running
through all GMs) from 5307 to 5280.

[bsalomon, 2013-09-11 20:02:43]

On 2013/09/11 18:50:31, JimVV wrote:
> This causes no change in GMs, and lowers the number of uploads (when running
> through all GMs) from 5307 to 5280.

lgtm

[jvanverth1, 2013-09-16 20:28:42]

Committed patchset #5 manually as r11303 (presubmit successful).