With this CL, we have the option to select a code output backend: GLSL, GLSL ES, or HLSL.

Note that we always emit the highest supported float precision for fragment shader due to anglebug 168.  Although this is a temporary solution, it's not against GLSL ES spec, because it's ok for implementation to upgrade precision.

Tested with WebGL conformance test suite, GLES2 conformance test suite (only failed 2/1198), and a few webgl demos, including worlds of webgl, aquarium, etc.

anglebug=81
test=translator emitting correct GLSL ES code when ES2 backend is selected.
Review URL: http://codereview.appspot.com/4550129

git-svn-id: https://angleproject.googlecode.com/svn/trunk@687 736b8ea6-26fd-11df-bfd4-992fa37f6226