Get rid of unnecessary GetSurfaceLevel/Release calls, move invariants out of the applyTextures loop, and fix the if in getSemanticIndex so we can avoid calling getMaximumCombinedTextureImageUnits. Gets donuts NaCl demo from 14->16 fps.

BUG=
TEST=webgl conformance tests

Review URL: http://codereview.appspot.com/5248057

git-svn-id: https://angleproject.googlecode.com/svn/trunk@787 736b8ea6-26fd-11df-bfd4-992fa37f6226