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.

minetest.conf.example

@@ -404,6 +404,12 @@
 #    type: enum values: 0, 1, 2, 4, 8, 16
 # fsaa = 0
 
+#    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.
+#    type: enum values: 0, 2, 3, 4
+# undersampling = 0
+
 #### Shaders
 
 #    Shaders allow advanced visual effects and may increase performance on some video cards.