[nacltoons] Add design resolution.

Adding a design resoltuion allows the game to be designed at
one resolution and run at another.

Also add brush clamping.

Review URL: https://codereview.chromium.org/12380036

git-svn-id: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1514 050acbb0-2703-11df-ab0a-9f3f633ae91d