Add tsdl-mixer

[jonahbeckford]

Since tsdl-* is the graphics tool we'll be recommending to Windows users, get tsdl-mixer working in Windows, at least in utop and bytecode.

Not native code though. Windows has open bug ... I think it is flexdll. When flexdll links OCaml programs on Windows it removes all shared libraries (ex. SDL2) that aren't explicitly used by the OCaml program. That means all ctypes programs, like tsdl-*, don't work in native code. cstubs would work, and so does adding at least one explicit OCaml-to-C call to SDL2 libraries. I did a fix for SDL2 in https://erratique.ch/repos/tsdl/tree/src/tsdl_stubs.c#n8 ... think this needs to be done for all the other tsdl libraries as well.

Follow-up to https://github.com/sanette/tsdl-image/pull/5#issuecomment-1449624451

cc: @sanette