add mat3 uniform setter in ofShader
add note recommending 4x4 mat for shader uniform
Merge branch 'master' into feature-shader-uniform-mat3
Merge branch 'master' into feature-shader-uniform-mat3

Conflicts:
	libs/openFrameworks/gl/ofShader.h
directly access ofMatrix3x3 member when setting uniform
Merge pull request #3177 from admsyn/feature-shader-uniform-mat3

Add setter for mat3 uniforms in ofShader