Fix for GL_HALF_FLOAT OpenGLES 2.0

This fixes undefined ```GL_HALF_FLOAT``` for OpenGLES 2.0 which was stopping compile.

- Fix for OpenGLES 2.0 missing define for ```GL_HALF_FLOAT```.

Note this ```GL_HALF_FLOAT``` / ```GL_HALF_FLOAT_OES``` will not work
on OpenGLES 1.1 so that case may still be undefined for earlier targets.

Also note worthy: It will also not work on Hardware Devices earlier than iPhone 4S or iPad 2 (and earlier can run OpenGLES 2.0)