use vertex-alpha for outline shader
use vertex-alpha for outline shader

This makes sure the outline fades out together with the rest
of the structure. Without this patch, the structure is rendered
in outline color (see #55).

CyrusBiotechnology/pv,biasmv/pv,Traksewt/pv,CyrusBiotechnology/pv,Traksewt/pv,yuhangwang/pv,biasmv/pv,yuhangwang/pv,Traksewt/pv,yuhangwang/pv,CyrusBiotechnology/pv,biasmv/pv,CyrusBiotechnology/pv,Traksewt/pv,biasmv/pv,yuhangwang/pv