Calculate sky color using raw brightness without gamma correction

[x2048]

Fixes #10258

In the calculation of sky color I've replace light-curve-corrected perceptive brightness in night/dawn/day detection rules with light-curve-agnostic linear brightness based on time_of_day.

Time points for dawn are preserved at 4:45 to 5:00, symmetric time for dusk.

To do

This PR is Ready for Review.

How to test

1. Add snippet from Client lighting parameters affect which sky colors are shown #10258 to devtest
2. Change light curve parameters (lighting_alpha, lighting_beta, lighting_boost, display_gamma ...) to any valid value (e.g. bright nights etc.)
3. Set time_speed to 576
4. Enter the game and set time to 4:00
5. Observe the transition of sky color from blue to green to red from 4:45 to 5:00, regardless of the light parameters.

[lhofhansl]

We can also pass both values in from the caller (game.cpp)?
(In that case we might want to rename time_brightness to adjusted_brightness - or something)

Generally there are more places where time_brightness and m_time_brightness are compared with fixed value. Do we need to look at those as well?

[sfan5]

In case I don't get to it here's what I think is important to test:

• take screenshots at various daytimes in 5.6
• take the same screenshots with this PR
• compare and make sure they're (nearly) identical