Restores stem length and stem-begin-position.

Restores stem length and stem-begin-position.

These two properties are used by the stencil and by the Y-extent
(unpure) callbacks to generate the stem.  This allows users to:

* Use length like they did prior to 2.15.9.
* Override Y-extent without there being an effect on the stencil.

This patch also makes minor fixes in the pure height estimations of
certain beamed notes.