[Bug 43893] Disable use of SHM extension through environment variable

https://bugs.winehq.org/show_bug.cgi?id=43893

--- Comment #2 from Panard  ---
Sure, but in our case, this is to disable only clients in a particular
container, not all the clients on the host.
Moreover, looking at the current code in winex11.drv/bitblt.c, if the extension
is disabled on the server, create_shm_image will still make a lot of work until
it gets kicked off by XShmAttach because the current implementation of XShm
still enable XShmCreateImage if the extension is disabled ; therefore disabling
the extension on the server will lead to very bad performances with wine.