Improve precision for vp computation

[toxieainc]

...to fix regression for at least SRally2 and HarleyD

fixes/improves #201

[dukeeeey]

Are these functionally the same?

float x = 2.0F / (right - left);
float y = 2.0F / (top - bottom);
float a = (right + left) / (right - left);
float b = (top + bottom) / (top - bottom);

float x = 2.0f / (right - left);
float y = 2.0f / (top - bottom);
float a = left*x + 1.0f;
float b = bottom*y + 1.0f;

Some games use off-axis projection, harley etc. Also a small request :) If you could put less changes in each pull request lol. It's hard to know what has really changed with all the factoring.

Also c++ has automatic type promotion. It's really not necessary to cast an int to a float when multiplying by another float. I mean, you can, but it happens automatically anyway. But maybe this is just style bitching :)

[toxieainc]

I personally prefer it like that, so that its clear whats happening. And in theory you can loose precision, depending on the integer value.

And the code is the same, yes. This way you can share more of the same potential error.

[toxieainc]

As for putting additional stuff into such PRs: Sorry, i will try to be more careful then.
I have a lot of local changes piled up, so if some are in the same file as my 'actual fix', and these are rather harmless, then i tend to just squeeze them in, too.