Hard-coded undersampling.

Adds uniform undersampling for the 3D rendered scene. GUI elements
are not undersampled, resulting in better playability for users
with low-performance platforms with readable fonts and formspecs.

The undersampling setting can be set to 0 (disabled), 2, 3, 4 pixels
which translates into a resolution reduction of x4, x9 or x16, and
is significant.

src/defaultsettings.cpp

@@ -108,6 +108,7 @@ void set_default_settings(Settings *settings)
 	settings->setDefault("show_debug", "true");
 #endif
 	settings->setDefault("fsaa", "0");
+	settings->setDefault("undersampling", "0");
 	settings->setDefault("enable_fog", "true");
 	settings->setDefault("fog_start", "0.4");
 	settings->setDefault("3d_mode", "none");