Keep track of the maximum used sampler to avoid expensive iterations in applyTextures and validateSamplers.

BUG=
TEST=webgl conformance tests

Review URL: http://codereview.appspot.com/5246061

git-svn-id: https://angleproject.googlecode.com/svn/trunk@795 736b8ea6-26fd-11df-bfd4-992fa37f6226