This is implemented by adding a new compile option SH_EMULATE_BUILT_IN_FUNCTIONS.  The emulated functions are names as webgl_originalName_emu so there will never be naming conflicts.

At the moment only three functions are emulated: normalize, abs, sign.  Also, the compile option will emulate all three.  However, the mechanism to emulate only a selected subset is also imeplemented.  It can be turned on easily.

ANGLEBUG=196
TEST=with this option, the failed test with abs.frag passes.
Review URL: http://codereview.appspot.com/4916043

git-svn-id: https://angleproject.googlecode.com/svn/trunk@738 736b8ea6-26fd-11df-bfd4-992fa37f6226