Add extended shared memory attach structure
This provides an extended version of X_ShmAttachFd, called
X_ShmAttachFdExt, which allows the client to specify two additional
parameters:
- The size of the mapping, in bytes.
- The offset that should be passed to
mmap, in bytes.
The intended use-case for this extension is when one needs to map
a drawable from a character special device, but it can also be
used with regular files if one needs to map with a nonzero offset.
Qubes OS needs the X server to map the /dev/xen/gntdev character
device, which represents memory shared from a different virtual
machine. This is currently accomplished with an LD_PRELOAD’d
shared library that overrides shmat, shmctl, and shmdt, and
I have written a version that instead overrides fstat and mmap.
With this extension, the preloaded shared library will not be required.