Revert gamma upper limit to 3.0 because that was raised based on
a misunderstanding and had no benefit. A sane upper limit is
needed as players on a competitive server tend to use the maximum.
Set gamma lower limit to 0.33 for consistency with 3.0.
Set sane limits on alpha, beta, boost and enforce these in code
to limit values entered in minetest.conf and to avoid easy cheating
by editing settingtypes.txt.
Improve documentation and 'readable' setting names.
Clarify that gamma does not significantly affect natural night light.
light.cpp: Various codestyle and comment improvements.
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.
