Fix Filter parsing

As mentioned in https://github.com/googlei18n/ufo2ft/pull/238:
We need to change the way filter strings are parsed in glyphLib. `Width:+30`, which should add 30 to the width, doesn’t mean the same as`Width:30` or `Width:=30` or `Width:=+30`, which should set the width to 30.

`Width:-30` should substract 30 from the width, while `Width:=-30` should set to the width to 0 since width cannot be negative (`RSB:=-30` and `LSB:=-30` do set RSB and LSB to -30).

The issue identifies filter-string parsing in glyphLib and references pull request #238; start by reading that discussion and locating the parser it identifies. Check the Width, RSB, and LSB examples, and consider the work done when additive and assignment forms produce the specified values, including the non-negative Width behavior.

Written by the indexing model from the issue text.