Hash table lookups are somewhat expensive, so cache a pointer to the current program for on a context. This gains about 3 fps (from 58) on a Native Client demo.

BUG=
TEST=webgl conformance tests

Review URL: http://codereview.appspot.com/5206042

git-svn-id: https://angleproject.googlecode.com/svn/trunk@786 736b8ea6-26fd-11df-bfd4-992fa37f6226