Removed forDistance parameter for mask conversions

It turns out to be unnecessary because distance conversions are as
simple as multiplication by a coefficient and wrapping their values
didn't make any sense. Thus the mask conversions will now be for
position only.
Pre-calculated widths, fixed distance conversions, encapsulated variables

Also, every method involving position may now accept values equal to the
width or height, simply referencing the value for the position right
next to it.
Flipped level rendering

It just didn't make sense to start the top of the level from the origin.
Now the entire level is rendered in the first quadrant. Tile helpers
have also been adjusted to account for this.
Added floor and ceiling helpers

They are the byproduct of trying to solve a frustrating bug with
GetMapSurface. Now there isn't much use for them, but who knows?