dynamic conpty loading

[Thomas Wolff]

The ancient conhost and also the initial conpty implementation of 
Windows were obstacles to using terminal feature from Windows applications.
There are nuisances like mangled or ignored character attributes, 
terminal reports not passed through etc.
While there is an updated Windows conpty layer being developed as part 
of the Windows terminal project, it is not yet deployed with Windows 11 
and will probably not be deployed with Windows 10 at all anymore 
(https://github.com/microsoft/terminal/issues/17452).
Various terminals had to fight with these problems and some found a 
solution.
According to 
https://github.com/saitoha/libsixel/issues/199#issuecomment-3181728693 
it is possible to inject a specific version of conpty so to supersede 
the Windows-installed conhost.exe.

I've tried to apply that to mintty by just preloading a conpty.dll 
before calling forkpty for the terminal child process but that has no 
effect.
The procedure seems to be:
try LoadLibrary("conpty.dll"), (if not successful, fallback to 
GetModuleHandle("kernel32") instead) to retrieve GetProcAddress for 
CreatePseudoConsole, ResizePseudoConsole, ClosePseudoConsole, and then 
somehow (?) use those while calling CreateProcess.
The latter is woven into the cygwin library and I don't think it's a 
good idea to clone that out of cygwin for a patched process creation in 
mintty.

My question/suggestion:
Can a cygwin mode switch to a selected conpty library instead of the 
default one please? My idea would be to configure this per process with 
the environment variable CYGWIN. Like its error_start= setting, it could 
have a conpty= setting to configure the filename to be used for conpty 
when starting a subprocess.
That would allow mintty to inject a modern conpty when calling a Windows 
application (e.g. wsl.exe).

Thomas