added MESA_GLX_FORCE_ALPHA env var

docs/VERSIONS

@@ -1249,6 +1249,7 @@ Mesa Version History
 	  (Justin Novosad of Discreet) (see config.h file)
 	- support for AUX buffers in software GLX driver
 	- updated glext.h to version 23
+	- new MESA_GLX_FORCE_ALPHA environment variable for x11 driver
     Changes:
 	- fragment fog interpolation is perspective corrected now
 	- new glTexImage code, much cleaner, may be a bit faster

docs/envvars.html

@@ -31,6 +31,7 @@ software driver:
 <li>MESA_GLX_FORCE_CI - if set, force GLX to treak 8bpp visuals as CI visuals
 <li>MESA_GLX_FX - set to either "fullscreen" for full-screen rendering,
   "window" to render into a window, or "disable" to disable the Glide driver.
+<li>MESA_GLX_FORCE_ALPHA - if set, forces RGB windows to have an alpha channel.
 </ul>