libGL: Make sure a valid value is returned for GLX_BIND_TO_MIPMAP_TEXTURE_EXT.

If the server didn't send a value, assume it's not supported.

A more generic solution might be better for this kind of problem, but an
attempt for this failed (see https://bugs.freedesktop.org/show_bug.cgi?id=9264)
and this allows compiz to work with drivers that support
GL_EXT_framebuffer_object.

src/glx/x11/glcontextmodes.c

@@ -333,7 +333,8 @@ _gl_get_context_mode_data(const __GLcontextModes *mode, int attribute,
 	*value_return = mode->bindToTextureRgba;
 	return 0;
       case GLX_BIND_TO_MIPMAP_TEXTURE_EXT:
-	*value_return = mode->bindToMipmapTexture;
+	*value_return = mode->bindToMipmapTexture == GL_TRUE ? GL_TRUE :
+	    GL_FALSE;
 	return 0;
       case GLX_BIND_TO_TEXTURE_TARGETS_EXT:
 	*value_return = mode->bindToTextureTargets;