Co-Authored-By: Sally Coolatta <tehrealsalt@gmail.com>
Co-Authored-By: James R <justsomejames2@gmail.com>
Co-Authored-By: Monster Iestyn <iestynjealous@ntlworld.com>
Co-Authored-By: katsy <katmint@live.com>

Place Frame Interpolation in "Experimental" video options header

This seems like an appropriate way to describe the feature for now.

Add smooth level platter under interpolation, `renderdeltatics`

`renderdeltatics` can be used as a standard delta time in any place,
allowing for smooth menus. It will always be equal to `realtics`
when frame interpolation is turned off, producing consistent
framerate behavior everywhere it is used.

Add smooth rendering to save select screen

Add smooth rendering to Record/NiGHTS Attack, F_SkyScroll

Ensure viewsector is accurate to viewx/viewy

This fixes a potential crash in OpenGL when changing between levels.

Ensure + commands get executed before map start

Always have precise_t defined

Fix misc dropshadow issues

Reset view interp...