[??-g23412a4f2] Interpolation issues with fps oscillations

[Edward-san]

If you play a map with the fps counter oscillating to values less than 35 and stay on a moving elevator, the visual bobs randomly up or down.

As an example, I used the kdizd elevator at z1m3 (warp to -128 -1155) and slowed down my CPU to the point that the fps oscillations <35 tics happen. It's useful if cl_capfps is enabled.

It happens with any renderer (cannot check with Vulkan on my linux machine, it's incompatible), but it's notable with the truecolor softpoly, as it seems slower for me.

In particular, if cl_capfps is enabled, the visual is constantly lower or higher than whenever cl_capfps is disabled.

Now that I think about it, it's been present for quite some time, but cannot check which version. Moreover, I can reproduce this on both Linux and OSX, not sure about Windows.