Add Mirror (Horizontal and Vertical) options

[cday]

Please give some consideration to adding support for the mirror functions provided in XnView MP Mirror - Horizontal and Mirror - Vertical.

This could allow implementation in a batch file of the mirror outputs requested in the post Another symmetry idea: Clone, flip horizontally and join.

[xnview]

you have xflip/yflip

[cday]

Maybe 'mirror' as used in the XnView MP view menu would be better for consistency, and also more intuitive or 'discoverable' ?

Not too late to add if the existing 'flip' options remain...