Created attachment 124148 [details] [review]
patch file for startx

XQuartz 2.7.9 stock install:
rcook@surface86 (~ ): glxgears
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  150 (GLX)
  Minor opcode of failed request:  3 (X_GLXCreateContext)
  Value in failed request:  0x0
  Serial number of failed request:  23
  Current serial number in output stream:  24
rcook@surface86 (~ ): glxinfo
name of display: surface86:18.0
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  150 (GLX)
  Minor opcode of failed request:  3 (X_GLXCreateContext)
  Value in failed request:  0x0
  Serial number of failed request:  17
  Current serial number in output stream:  18
rcook@surface86 (~ ):

XQuartz 2.7.9 with +iglx added to serverargs on line 112 of /usr/X11R6/bin/startx
defaultserverargs="$defaultserverargs +iglx"

Works perfectly.
name of display: surface86:18.0
display: surface86:18  screen: 0
direct rendering: No (If you want to find out why, try setting LIBGL_DEBUG=verbose)
server glx vendor string: SGI
server glx version string: 1.4
etc etc etc.

Attached is a patch file.  After patching startx, users can then do this: 
defaults write org.macosforge.xquartz.X11 enable_iglx -bool true