implements GLES2 ofDrawBitmapString

* we're using a built-in shader to draw ofBitmapStrings.
* tested  GLES2 + GLES1 on iPad, iOS 6.1.2
* tested GL2.0 + GL3.2 on OS X (10.8.2) [ati], Linux (arch) [nvidia]

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>