Fix for GL_HALF_FLOAT OpenGLES 2.0

- 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
It will also not work on Devices earlier than iPhone 4S or iPad 2.
Merge pull request #3401 from danoli3/ofGLUtils-fix-iOS-OpenGLES

Fix for GL_HALF_FLOAT OpenGLES 2.0
Merge branch 'master' of https://github.com/openframeworks/openFrameworks.git into rexgo