This commit performs three related cleanups:
* The slvs library no longer uses explicit platform initialization
(which drags in the side effects of InitPlatform that are not
desirable in a library). Instead, it just ensures that it has
the temporary heap, which is what it was callingInitPlatform for.
* InitPlatform is simplified and moved to platform.cpp, next to
other path related functions.
* InitPlatform is renamed to InitCli and is called from InitGui
implementations. GUI toolkits sometimes have options they use
internally (that's the case for for GTK and Cocoa at least),
and we shouldn't try to parse those as a file to open.
When drawing the graphics window, we flush it twice: once to draw
the geometry, and another time to draw the UI overlay (toolbar,
selection marquee, and FPS counter). Calling glFinish() each time
is (on most platforms) just pointlessly slow, but on macOS Catalina,
without offscreen rendering, it causes the toolbar to flicker.
Instead of calling glFinish() twice per frame in that case, call
glFlush() twice and then glFinish() once we really are done.
This is currently necessary to get repeatable results when exporting
assemblies as a part of a batch process, since the mesh geometry in
imported files is not regenerated for export.
