By allowing to specify a separate source bitmap when calling Filter::apply
the same filter can be applied to multiple areas, and also doesn't need
to use a temporary bitmap. This also enables us to apply the filter to
multiple regions properly, even if they are (almost) adjacent.
If no separate source bitmap is supplied then a temporary bitmap is still
necessary.
By moving the Bitmap and Rect out of Filter::Parameters we can re-use
the parameters more efficiently, allowing the filter to be applied
to many bitmaps without having to re-create the filter every time.
Add an overload of GenericConvolutionFilter::apply that can be used
with a GenericConvolutionFilter::ApplyCache instance to avoid having
to allocate a temporary bitmap every time the filter is being applied.
