Add ascender/descender proto defs, logic, & tests

[jseppi]

Closes #11

• Adds ascender and descender fields to glyphs.proto to match those in Expose ascender/descender metadata properties in protobuf node-fontnik#160
• Adds new logic to handle composite ascender and descender as described in Handle fontstack ascender/descender metadata #11:
  • If all fonts have the same value for ascender or descender, then the composite will have that value.
  • If any font has a different ascender or descender value, then the composite will have its value set to 0 (which will enable falling back to the hard-coded default values in mapbox-gl-js and mapbox-gl-native).
• Adds a few new test fixture PBFs that were built with the node-fontnik version in Expose ascender/descender metadata properties in protobuf node-fontnik#160 so that they have ascender and descender values set

@springmeyer for review, please

cc @tristen

[springmeyer]

PR seems straightforward, so 👍

If any font has a different ascender or descender value, then the composite will have its value set to 0 (which will enable falling back to the hard-coded default values in mapbox-gl-js and mapbox-gl-native).

I have some questions about this, will comment at #11