Use perfect fixed interval ticks/physics frames or, by default, integer multiples

[AltimorTASDK]

This adds support for fixed tick deltatime and multiple ticks per render frame. cl_mergeticks (default 1) disables simulating multiple ticks per frame and instead ticks only once, consuming the highest possible integer multiple of the tick interval from the accumulated time, avoiding slowdown on PCs that can't handle 72 tick at the expense of determinism by default.

Input is sampled once per frame, but mwheel inputs, wait, mouse strafe, m_filter, and CL_KeyState should all be properly accounted for when running multiple ticks/usercmds per frame.