smooth transition on primary/alt buffer change (maybe even on clear-screen)

[christianparpart]

trafficstars

• [x] (VT backend) ability to explicitly render primary/alt screen, regardless of what is currently active. #472
• [ ] (frontend) render to texture (refactor screenshot functionality for this)
• [ ] (frontend) pass rendered texture of alternating screen buffer to shaders (required for smooth transitioning between the two).