cad/openvsp: update to 3.18.0

Add USES=xorg to complement USE_XORG

ChangeLog: http://openvsp.org/blogs/announcements/2019/09/03/openvsp-3-18-0-released

Features:

• New anthropomorphic pose-able human model
• More sets by default (20)
• Blue and green highlights for wing/fuse/stack editing
• Convert non-adjustable sliders to adjustable
• Improved Cpslice visualiation
• Other small cleanups

Fixes:

• Fix some API Error messages
• Don't draw Prop XSecs when in disk mode
• Fix memory leak