* Reverts 272dfa762d, but is a fresh implementation.
* I doubted anyone was doing this but apparently not.
* Exhumed needed a bit more work as it has very specific underwater checks.
* I tried to make this work in 2020 but its been nothing but headache. It's something for the game to control via a flag, not the backend to determine.
* For SW, this restores its original implementation.
* Since all velocities are floated, we can now take each game's keymove scaling into its ticker and use a consistent keymove size, making non-mouselook and controller input truly identical across the games without having to try and fudge any values.
* Remove fix16.h/cpp and utilise library from m_fixed.h.
* Extend m_fixed.h with two inline functions for int to/from float operations.
* Replace fix16_floor operations with those from xs_Float.h
* Replace multiple Q16.16 conversions from 0 to just be 0.
* Replaced all found in-game bit-shifts and multiplications/divisions with inline functions from m_fixed.h
* Replaced many casts of FRACUNIT as double in SW's panel.cpp as it is converted to double by way of type promotion.
* Fixed missed precision fixes in SW's panel.cpp where some types weren't declared correctly.
* Replaced 100+ `Cos()/Sin() >> 16` operations for Blood with inline functions `CosScale16()/SinScale16()`.
None of this was really working anymore, but will have to be rethought once a network stream exists again.
But even then, this is better sent as a separate command, and in MP only.
Currently the bit fields are still separate and they have to be merged, but for now the added memory does not matter.
Having this structure in the common parts will allow work on consolidating the input code, though.
