This is not complete, it does not implement 3D solid drawing of
envelope bones. 2D wire is hence always drawn for now.
Some notes:
I did not try to implement the 'capsule' approach suggested by @fclem, because:
1. I spent enough time on this already, and finally got something working.
2. I managed to get rid of geometry shader completely.
3. Current approach allows us to use same shader for
distance outline and envelope wire.
It's working fine, except for one glitch - superpositions of envelope
outlines do not work as expected, not sure what's wrong here, tried to
disable zbuff, enable GL_BLEND, no luck so far...
I think we need our own 'background' drawpass to get them working (also
to avoid them drawing over the wire lines).
3c799ba801