This created weird cases in logs which are very hard to understand. The
one which really got me was this:
```
[runtime] 2023-08-13 07:48:27 [verbose]: Invalidating working beatmap cache for unknown artist - unknown title (Dummy)
```
Which looks like a dummy working beatmap was invalidated, but it turns
out that's just the local user which was populated when creating a new
local beatmap.
Closes#24503.
The problematic reset of `lastParticleAdded` has been moved to a place
that should hopefully (a) not cause such problems and (b) be much more
explicit about what's happening.
Closes#24514.
The original PR was _supposed_ to be handling this, and had code that
_looked_ like it would handle it, but was in fact subtly wrong, as the
union of `RectangleF.Empty` (initial value of `area`) and an arbitrary
other rectangle is _not_ the second rectangle, but rather a rectangle
that contains _both_ the second rectangle and the point (0,0),
thus leading to the bound calculation fail.