Fix superflous shader setting updates

[ShadowNinja]

This can improve rendering performance by ~40%. (very hardware dependent)
Rebased version of #4530 with the following changes:

• Free ShaderCallbacks instead of just leaking them.
• Replaced the Sky** thing with a setSky() thing. It's still a bit ugly, but it works and it's maybe a little less ugly. (If you don't like it please offer a better solution)
• Removed read of uninitialized memory.
• Some minor style things that kwolekr noticed.

[Zeno-]

Since it's a WIP, I don't expect the memory-related problems have been addressed yet?

For the record, if a "factory" is the best approach (and I can't really think of a better one, but it seems confusing) then I'm ok with that.

Edit: SetSky() is similar to how I did it. I'm fine with that approach; it doesn't confuse things.

[Fixer-007]

It is 5th reincarnation of this PR, I hope you get it right this time once and for all. I guess I will use it daily.

[ShadowNinja]

@Zeno-: The compare with uninitialized values has been fixed, the memory leak has not been fixed (yet).

[ShadowNinja]

@Zeno-: Aaaaand fixed. (although I didn't have a few weeks to spare to wait for a valgrind run for confirmation...) This should be ready now.

[Zeno-]

👍

[Fixer-007]

Congratulations to @gregorycu and everyone involved in this.

[gregorycu]

Thank you @Fixer-007 . You were as involved as anyone else, so you should be pretty happy too!

[lhofhansl]

It's awesome to see this in.
My son has my old laptop with an ATI card (can't check which one right now), and it feels noticeably faster there.