Fixed|Renderer: Crash when drawing particles (jXCCP)

Ensure that the active set of generators is known when beginning
rendering of a map.

A crash occurred because the generators were set too late; the
renderer was trying to mark which generators are visible before
knowing which map's generators to use.
Cleanup|Client|libgui: Use "has" instead of "have"

Mostly a stylistic choice, although 'has' usually is more
grammatically correct.