rename programmable GL shader attributes

## restores variable name "color" for default ProgrammableGL GLSL shader vertex color attribute 

(does not affect any other rendering pipelines)

Renames "uniform vec4 color" -> "uniform vec4 drawColor", this allows us to use "color" for the shader vertex attribute parameter, and drawColor for the shader uniform parameter.

* "color" is the vertex colour, while
* "drawColor" is set through the  Material color / the current openFrameworks draw color 

+ Also stringifies the OpenGL default shaders for easier legibility.
+ Also fixes a typo within ofMatrixStack