Software renderer: Add linear interpolation of fog range adjustment factors.
Remove the flawed, unused attempt to manually emulate Gecko codes.

Parsing Gecko codes (in any manner) is much like parsing HTML with regex
- that w̷a̶y̸ l̵i̷e̴s̵ m̴̲a̵͈d̵̝n̵̙ę̵͎̞̼̙̼͔̞͖͎̝s̵̨̬̱͍͓͉̠̯̤͙̝s̷͍̲̲̭̼͍͎͖̤̭̘.  Luckily, with the embedded codehandler.bin,
the monstrosity may remain at only one implementation.  Anyway, removing
the inserted_asm_codes thing probably speeds up the interpreter a bit.
Add a non-tiny warning about CPUs that will silently desync.
Revert "Jit64: use AVX instructions in some places"

and the fixes afterwards

This reverts commit ff917897731d400a8e71c1480425733e651126ee.
This reverts commit bcefa880e4d062121df9ab8b831170ec3a689a50.
This reverts commit 710a0ff435c8593914c6eb074b6bc4d1985ae8c5.
This reverts commit ddaf29e039d1736be1ca48c1c0b6af1c8d4ec47d.
Jit64: load second operator for double intrutions

SSE do support non-vector instructions, but they _all_ overwrite the dest register
if the src location isn't a register. (wtf?)
So we have to load the src into a temporay register :(
Merge branch 'revert-avx'
Fix the ZLib in Externals. The CMakeLists file looked like it was copy-pasted from ZLib itself. It didn't work for various reasons.