Fixed bug: If texture gamma is changed from the default; during start…

…up GL_DoUpdateTexGamma is called before the file manager has loaded PLAYPAL and thus LoadPalette will fail. Check whether the texture manager is online before attempting to do any updating (if its not, we don't need to do an update anyway).

doomsday/engine/portable/src/gl_texmanager.c

@@ -3013,8 +3013,11 @@ void GL_TexReset(void)
  */
 void GL_DoUpdateTexGamma(cvar_t *unused)
 {
-    GL_TexReset();
-    LoadPalette();
+    if(texInited)
+    {
+        GL_TexReset();
+        LoadPalette();
+    }
 
     Con_Printf("Gamma correction set to %f.\n", texGamma);
 }