array index clamping to use the clamp intrinsic. This works more
reliably on pure OpenGL ES devices and on Windows.

Added a mechanism in ShBuiltInResources to choose the strategy for
array index clamping.

BUG=none
TEST=various out-of-bounds array indexing tests and various WebGL content
Review URL: https://codereview.appspot.com/7194051

git-svn-id: https://angleproject.googlecode.com/svn/branches/dx11proto@1798 736b8ea6-26fd-11df-bfd4-992fa37f6226