Fix mutation of global state 'InvalidRect64'/'InvalidRectD' by 'GetBounds' variants.

[ClaasJG]

Hi,

Clipper.GetBounds changes the state of InvalidRect64 and InvalidRectD.
Therefor GetBounds accumulates the biggest bounds over the lifetime of the program and does not compute the requested bounds for the requested path.

While this can be worked around by the user (because those fields are mutable and public)
it makes the static utility functions dependent on global state and therefor prohibits the usage in a multi threaded context.

I think the same bug (or at least absolutely annoying and unexpected behaviour) can be found in the original C# implementation. (EDIT: The original implementation implements Rect64 and RectD as struct and therefor as ValueType. Which means this port differs currently in behaviour. )

Have a nice day

-ClaasJG