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.

builtin/settingtypes.txt

@@ -362,6 +362,11 @@ texture_min_size (Minimum texture size for filters) int 64
 #    when set to higher number than 0.
 fsaa (FSAA) enum 0 0,1,2,4,8,16
 
+#    Undersampling is similar to using lower screen resolution, but it applies
+#    to the game world only, keeping the GUI intact.
+#    It should give significant performance boost at the cost of less detailed image.
+undersampling (Undersampling) enum 0 0,2,3,4
+
 [***Shaders]
 
 #    Shaders allow advanced visual effects and may increase performance on some video cards.