optimises matrix ops

this gets rid of one GL matrix operation + one unnecessary gl state change for any ofDrawBitmapStringHighlight draw operations which are not OF_BITMAPMODE_MODEL

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
Merge pull request #2216 from tgfrerer/fix-drawbitmapstring-highlight

ofDrawbitmapStringHighlight: optimises matrix ops