Allow to specify game start time

[lwho]

Some time ago someone complained that you start at night in Los Angeles.

This PR allows to specify the game start time in the Lua function Game.StartGame and sets the start time for Los Angeles to start at sunrise.

[johnbartholomew]

Specifying game start time is fine, but I'm not sure about the changes to Frame.

We've been able to specify rotational phase offset since #1506 was merged back in September 2012. If you want it to be daytime in LA at game start, all you need to do it adjust Earth's rotational_phase_at_start value in data/systems/00_sol.lua.

[robn]

Or just choose a different start point. We've done it before. Nothing special about LA!

[lwho]

Actually I do not care at all about LA. It is just a good example.

Fiddling with rotational_phase_at_start or changing the start location works only for a custom system, but not for generated systems. I have a starting point in the outskirts of the settled areas an I preferred to see the wonderful planet I chose at start.

Moreover, I considered it a bug that the rotation resets to rotation_phase_at_start whenever you enter the system (not only at game start). So, while it is pitch-dark at LA at game start at 00:00, it could be noon at the same time when you revisit LA later when jumping out and into the system. I think, this is not the case for the orbits, only for the self rotation (but it is some time ago that I looked into this and wrote the code).

Last but not least, this iterative multiplication with the rotation matrix accumulates rounding error astonishingly fast. So, even if you stay longer in the solar system, you may notice day drift.

[johnbartholomew]

Fiddling with rotational_phase_at_start or changing the start location works only for a custom system, but not for generated systems.

I'd rather it was easy to make custom modifications to a generated system (or otherwise specify the system); fiddling with the starting time seems like a hack if that's why you're doing it. But that's a problem for some other PR in the future.

Moreover, I considered it a bug that the rotation resets to rotation_phase_at_start whenever you enter the system (not only at game start).

Agreed, that should be fixed.

Last but not least, this iterative multiplication with the rotation matrix accumulates rounding error astonishingly fast. So, even if you stay longer in the solar system, you may notice day drift.

Agreed, that should be fixed.

[johnbartholomew]

Why is this needed?

[lwho]

Because this constructor calls the Space(Game*, const SystemPath &) constructor which calls Space::GenBody(SystemBody *, Frame *) which calls MakeFrameFor(SystemBody *, Body *, Frame*) which needs the current time (Pi::game->GetTime()) for setting up the orientation of rotating frames.

Game::CreateViews seems to use the same hack.

Well the if (!Pi::game) is probably not needed ;)