Two new layer-agnostic filters: Vertical Blur and X-Swapped Polar Convert

These were needed for a modified zoom blur algorithm; also, vertical
blur may be retrofitted (alongside horizontal blur) to make box blur
much faster
Improved zoom blur

Zoom blur now provides two variants: "modern" and "traditional".
Traditional works similar to radial blur (polar convert, unidirectional
blur, rect convert), and the output is more along the lines of other
photo editors.  I've tried to mitigate damage loss due to the repeat
polar/rect conversions; the new XSwapped polar convert method helps with
this.

Note that the "modern" version allows for zooming-out, while
"traditional" does not.
Allow user to swap x/y during polar conversion

Swapping will yield less data loss if the image is wider than it is
tall.