Profiling shows that creating and destroying vertex declarations is extremely expensive, so we can keep a 16-element cache around to speed that up.

BUG=
TEST=JSGameBench

Review URL: http://codereview.appspot.com/4358051

git-svn-id: https://angleproject.googlecode.com/svn/trunk@609 736b8ea6-26fd-11df-bfd4-992fa37f6226