advance width support including:

* exposure to CPS
* ability to update using CPS
* updated CPS value respected during export to UFO

advance height is also added.
I was starting to try to play around with getting glyphs to be non square on screen. So advanceWidth would make a difference to how wide (at same scale) a glyph was rendered. Since that was the only var and is unsed I removed it form this PR.