Truckload of updates

[madebr]

Highlights:

• blip in the map is working:
  
• save + load screens + logic "working".
  I don't guarantee any compatibility with the original carmageddon savegames.
  So create a backup before using it, is advised...
  
• APO keys working (and not working in single player campaign):
  
• Various implementations

Overall progress:
55% of all functions implemented. (1900 completed / 3471 total)

[dethrace-labs]

While this is awesome work, as usual(!), this kind of PR is a little difficult for me to handle.

PRs that implement a feature or behavior end-to-end are easier to
a) test before merging
b) because we can test them, that ensures all the code going in to main doesn't just compile, but actually executes correctly (as best as we can tell).

This PR has some things that should be easy to merge:

• Load / Save full end to end
• Partial Map rendering (we have enough to see that what you've done so far is working)

Things that I don't mind as extras in the PR:

• Extra BRender functions that might/might not be required specifically for dethrace
• Tidying (replacing 0's with NULL, adding STUB calls where appropriate, removing incorrectly inlined function calls etc etc

Things that make it hard to merge:

• Functions implemented which either aren't called yet, or are part of a feature that is not completed end-to-end, so I have no way to sanity check except by looking at each function myself:
• Examples: StripBlendedFaces, the crush code, the sparks code, the shrapnel code, flame code, smoke code, etc.
• MakeCarIt is a good example of what I'm trying to say - that code is not going to be used until we have network play. When we start on network play, that would be a good time to implement it, because it will get used and tested. Right now it has an implementation which we don't know is right or not. As we add functionality, I want to be able to trust the code that is already there. It makes debugging so much nicer when we don't hit bugs in a function that was implemented earlier but never validated.

I'm torn, because I see all your good work, but I want to get into the habit of merging full features without so much random extras

[dethrace-labs]

I normally use BrVector3LengthSquared here - probably BR_SQR3 could be removed from the public brender include

[madebr]

I used this one because I found it weird to cast a row of a matrix to a vector, but it makes sense to do it.
So I'll do that from now on.

[dethrace-labs]

Going to merge this now