cad/gmsh: update to version 2.15.0

Upstream changes:

• fixed several regressions (multi-file partitioned grid export, mesh subdivision, old compound mesher)
• improved 2D boundary layer field & removed non-functional 3D boundary layer field
• faster rendering of large meshes

Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D8725