Remove a bunch of code from XFixes SProcs

`stuff->length` can be swapped once, rather than having to be swapped in
each sproc.
5 jobs for xfixes-sproc-version-check in 7 minutes and 19 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-buster #12345622

00:00:13

 
  Build And Test
passed autotools #12345623

00:07:06

passed meson #12345624

00:03:25

passed meson-noglamor #12345625

00:03:36

passed mingw-cross-build #12345626

00:00:45