Fix multisampling on X11

Setting SDL_GL_ACCELERATED_VISUAL was disabled for ioq3 SDL 1.2
for other reasons. However, it causes creating GL context to fail
if multisampling is enabled on X11 for both SDL1.2 and SDL2.

Tested using nVidia proprietary driver on Debian 7.

code/sdl/sdl_glimp.c

@@ -407,9 +407,11 @@ static int GLimp_SetMode(int mode, qboolean fullscreen, qboolean noborder)
 
 		SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );
 
+#if 0 // if multisampling is enabled on X11, this causes create window to fail.
 		// If not allowing software GL, demand accelerated
 		if( !r_allowSoftwareGL->integer )
 			SDL_GL_SetAttribute( SDL_GL_ACCELERATED_VISUAL, 1 );
+#endif
 
 		if( ( SDL_window = SDL_CreateWindow( CLIENT_WINDOW_TITLE, x, y,
 				glConfig.vidWidth, glConfig.vidHeight, flags ) ) == 0 )