Qt+VTK+OpenGL rendering fixes

Improvements to address following issues:

• #17740 (closed) (no support for crystal eyes)
• #18513 (closed) (bad screenshots on macos) -- verified on 10.13.6 with Qt 5.12.1 + retina
• #18830 (closed) (view creating undo)
• #18446 (closed) (save animations broken)
• #18516 (closed) (rework backbuffer / offscreen rendering code)
• #18678 (closed) (charts don't have anti-aliasing on macOS) -- verified on 10.13.6 with Qt 5.12.1 + retina
• #17394 (closed) (render view does not fill window after saving smaller screenshot) -- verified on 10.13.6 with Qt 5.12.1 + retina
• issue noticed in !3098 (merged) (Save all views seems to not be working)
• #18858 (closed) (segfault)

Outstanding issues that will be addressed separately:

• #18828 (closed) (font scaling on retina)

Depends on vtk/vtk!5214 (merged)