add read-only access to programmable GL matrix stack

in 'classic' OpenGL 2.0, you could call:

````glGetFloatv(GL_MODELVIEW_MATRIX, matrixPtr);````

To read back the current matrix state from the GPU.

This is not possible with modern OpenGL, since the matrix stack is now client-side.

With this PR, we get this functionality back into the programmable GL renderer pipeline. Query the current matrix state as in:

````ofMatrix4x4 currentModelViewMatrix = ofGetGLProgrammableRenderer()->getModelViewMatrix();````

Since these are read-only methods, they are marked ````const````

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>