added MESA_GLX_ALPHA_BITS env var

docs/VERSIONS

@@ -1419,6 +1419,7 @@ Mesa Version History
 	- added fast XOR-mode line drawing optimization
 	- radeon: add support for all 3 tmus, GL_ARB_texture_cube_map
 	  and GL_EXT_fog_coord
+	- MESA_GLX_ALPHA_BITS env var for xlib driver
     Changes:
 	- removed GL_HP_occlusion_test (use GL_ARB_occlusion_query instead)
     Bug fixes:

docs/envvars.html

@@ -37,6 +37,7 @@ software driver:
   "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.
 <li>MESA_GLX_DEPTH_BITS - specifies default number of bits for depth buffer.
+<li>MESA_GLX_ALPHA_BITS - specifies default number of bits for alpha channel.
 </ul>