Fix these memory leaks:

==30021== 128 bytes in 1 blocks are definitely lost in loss record 230 of 302
==30021==    at 0x483577F: malloc (vg_replace_malloc.c:299)
==30021==    by 0x2EF89C: init_visuals (xf86glx.c:390)
==30021==    by 0x2EF89C: __MESA_initVisuals (xf86glx.c:541)
==30021==    by 0x17C922: GlxInitVisuals (glxext.c:317)
==30021==    by 0x218E73: fbInitVisuals (fbcmap.c:668)
==30021==    by 0x20BEB1: fbFinishScreenInit (fbscreen.c:229)
==30021==    by 0x20C275: fbScreenInit (fbscreen.c:273)
==30021==    by 0x1E0317: nxagentOpenScreen (Screen.c:1357)
==30021==    by 0x16D848: AddScreen (dispatch.c:4171)
==30021==    by 0x1DB7FF: InitOutput (Init.c:396)
==30021==    by 0x14DB12: main (main.c:279)
==30021==
==30021== 3,072 (192 direct, 2,880 indirect) bytes in 1 blocks are definitely lost in loss record 290 of 302
==30021==    at 0x483577F: malloc (vg_replace_malloc.c:299)
==30021==    by 0x2CCCC7: _gl_context_modes_create (glcontextmodes.c:364)
==30021==    by 0x2EF87C: init_visuals (xf86glx.c:381)
==30021==    by 0x2EF87C: __MESA_initVisuals (xf86glx.c:541)
==30021==    by 0x17C922: GlxInitVisuals (glxext.c:317)
==30021==    by 0x218E73: fbInitVisuals (fbcmap.c:668)
==30021==    by 0x20BEB1: fbFinishScreenInit (fbscreen.c:229)
==30021==    by 0x20C275: fbScreenInit (fbscreen.c:273)
==30021==    by 0x1E0317: nxagentOpenScreen (Screen.c:1357)
==30021==    by 0x16D848: AddScreen (dispatch.c:4171)
==30021==    by 0x1DB7FF: InitOutput (Init.c:396)
==30021==    by 0x14DB12: main (main.c:279)

The problem here is that GlxInitVisuals is called twice. First via
fbScreenInit and then again via nxagentInitGlxExtension. We remove the
first one to ensure the code in nxagenOpenScreen works as initially
intended.

There's an xorg upstream patch that does the same
(7d74690536b64f7b8e8036507ab7790807349c50), but it also cleans up
other stuff we do not even have in out source (yet?).