Add extended shared memory attach support

This provides an extended version of ‘create_pool’, called
‘create_pool2’, which allows the client to specify a 64-bit offset in
the file to map at.  As Wayland does not support 64-bit integers, the
offset is passed as two 32-bit numbers.

The intended use-case for this extension is when one needs to map a
surface 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 Wayland compositor to map the ‘/dev/xen/gntdev’ character device,
which represents memory shared by a different Xen virtual machine.
Currently, this can be accomplished by opening a separate instance of
‘/dev/xen/gntdev’ every time, but that is slightly wasteful.  Until
recently, it also on undocumented behavior in the kernel driver.

This also requires libwayland-server to be built with 64-bit off_t,
which should be supported on any reasonably modern system.  A
‘_Static_assert’ will trip if off_t is not large enough.

This also forbids resizing a pool of version 3 or later that is
currently in use.  On non-Linux systems, supporting pool resize requires
holding the file descriptor open, which can lead to file descriptor
exhaustion in the compositor.  This change allows libwayland-server to
close the file descriptor once the pool is first mapped.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>

meson.build

@@ -26,6 +26,7 @@ compiler_flags = [
 	'-Wstrict-prototypes',
 	'-Wmissing-prototypes',
 	'-fvisibility=hidden',
+	'-D_FILE_OFFSET_BITS=64',
 ]
 
 cc = meson.get_compiler('c')