Add silhouette edge drawing

[Tom Goddard]

Add silhouette edge drawing.

Possible implementation: write depth buffer to texture, then draw full window black rectangle using this depth shifted backwards by the minimum depth jump for an edge to show. Use 4 shifts one pixel along x and y.

[Tom Goddard]

Done.

The new silhouette command and shortcut "se" allows enabling silhouette edges:

silhouette on|off [thickness pixels] [color r,g,b|name] [depthJump j]

This edge drawing is faster than Chimera 1 because it uses a single drawing of the scene, while Chimera 1 draws the scene two times using glPolygonMode and stencil buffer operations. This makes Chimera 1 render silhouettes at half frame rate, while the new method renders just one pass plus a several texture draws on full-window quads which are very fast. For complex scenes the new code is expected to render silhouettes at approximately the same frame rate as no silhouettes.