added Caching Mouse Coordinates to reduce redundant calls

[Chewbakka-Wakka]

Reduces redundant calls, by Caching of mouseX() and mouseY() at the start of processMouseClickInput

Why do?
mouseX() and mouseY() functions are called multiple times per frame, and each call recomputes the mouse coordinates.

Note on explicit assignment to help make intent clearer.

Some comments re-shuffled.

held off on changing mousePos -> Vector2i. Seen elsewhere though.

[Chewbakka-Wakka]

mouseInBox may be another candidate

[past-due]

Fortunately, mouseX() and mouseY() already return cached coordinates. (But good instinct to check for areas where caching could benefit.)

Also, in the future, please try to avoid unrelated changes - such as to whitespace or formatting - when opening a PR. It makes it harder to review and merge.