Redo light.cpp.

Remake the light_decode_table.

The table starts out without pre-filled in values since those
are always discarded by the code apparently. We calculate a
pseudo curve with gamma power function, and then apply a new
adjustment table.

The adjustment table is setup to make the default gamma of 2.2
look decent: not too dark at light level 3 or so, but too dark
at 1 and below to be playable. The curve is much smoother than
before and looks reasonable at the whole range, offering a
pleasant decay of light levels away from lights.

The `display_gamma` setting now actually does something logical:
the game is darker at values below 2.2, and brighter at values
above 2.2. At 3.0, the game is very bright, but still has a good
light scale. At 1.1 or so, the bottom 5 light levels are virtually
black, but you can still see enough detail at light levels 7-8,
so the range and spread is adequate.

I must add that my monitor is somewhat dark to begin with, since
I have a `hc` screen that doesn't dynamic range colors or try to
pull up `black` pixels for me (it is tuned for accurate color and
light levels), so this should look even better on more dynamic
display tunings.

minetest.conf.example

@@ -508,7 +508,7 @@
 #    Adjust the gamma encoding for the light tables. Lower numbers are brighter.
 #    This setting is for the client only and is ignored by the server.
 #    type: float min: 1 max: 3
-# display_gamma = 1.8
+# display_gamma = 2.2
 
 #    Path to texture directory. All textures are first searched from here.
 #    type: path