Optimize the decoders a bit

[aras-p]

The actual changes are fairly simple, and most of them are like "instead of doing work at byte level, do it at integer level".

In my tests, on windows (vs2022, ryzen 5950x):

• BC1 821->1327 Mpix/s
• BC3 516->694
• BC6H 65->85
• BC7 91->143

On mac (clang 13, M1 Max):

• BC1 804->2037
• BC3 585->1062
• BC6H 63->76
• BC7 113->212

With the speed bump, this makes it one of the fastest BCn decoders out there, actually (with some exceptions in some formats). I plan to write about this more somewhere, but here's a sneak peek (higher numbers are better; bcdec is upstream repo, bcdec_opt is with this PR)

[iOrange]

Thanks! The bits pulling optimization is quite clever, I went the lazy way, got really tired hard-coding all those partitions tables :)