Adding 4-channel output support and fixing 4-op mode edge case bug

[Kagamiin]

Description

• OPL3_Generate has been refactored into OPL3_Generate4Ch.
  • OPL3_Generate4Ch contains the original logic for sample generation, but modified to output into 4 channels (2 DACs) like the real chip does
  • Added shims for OPL3_Generate, OPL3_GenerateResampled and OPL3_GenerateStream to maintain compatibility with existing applications that expect 2 channel output
  • Tested on a hacked version of Furnace Tracker, 4 channel output works as expected, 2 channel output still works the same.
• Fixed bug where setting the 4-op flags would not immediately change the channel connections until a write to base 0xC0 was issued (bug encountered in Adlib Tracker II when previewing instruments, confirmed by ValleyBell to not occur in real hardware)

Notes

• Added inline attribute to OPL3_Generate4Ch; checked on Compiler Explorer and with -O3 optimizations on GCC this results in OPL3_Generate (and GenerateResampled) being generated with very similar code to the original which should result in almost no performance impact for apps that only use 2 channel output.
  • If -O2 is used, GCC chooses to not inline the code (as if the inline hint wasn't there). Did not check other compilers.

[Kagamiin]

Some of the line endings were mangled in the original file and this caused some trouble for me when generating the commits. I ended up converting the existing CRLF endings to LF, hence the weird changes in comments. Oh well.

[nukeykt]

Thanks!