[PATCH 0/2] Don't use xshmfence unless supported by the driver

[Keith Packard]

This makes the xshmfence stuff only used if the driver specifies it,
allowing drivers to specify other FD-passing based fences, and also
avoiding errors if a driver doesn't initialize any FD-passing based
fences.

Without these changes, the X server will crash when an application
creates an xshmfence object and passes that to the X server to a
driver that has no DRI3 FD passing support.

-keith