Fixes ArcticaProject/nx-libs#301, #631

Backport from xorg-xserver:

  commit ca82d4bddf235c9b68d51d68636bab40eafb9889
  Author: Eric Anholt <eric@anholt.net>
  Date:   Fri Aug 31 13:00:23 2007 -0700

    Bug #7186: Fix an excessive request size limitation that broke big-requests.

    MAXBUFSIZE appears to be a leftover of some previous time.  Instead, just
    use maxBigRequestSize when bigreqs are available (limiting buffers to ~16MB).
    When bigreqs are not available, needed won't be larger than the maximum
    size of a non-bigreqs request (256kB).