Selectable registration resolution

[krupkat]

Add an option to select registration resolution (in MPx) for the stitching algorithm. The images are downscaled to this size for finding matches between images.

The default value is 0.6MPx and that could be an issue e.g. if stitching two photos with electrical wires spanning over both of them.

Howto

• add a registration_resolution in: https://github.com/krupkat/xpano/blob/dee2e45635bf9ebaa88b8f098bf8378765f653be/xpano/algorithm/options.h#L82
• call SetRegistrationResolution in https://github.com/krupkat/xpano/blob/dee2e45635bf9ebaa88b8f098bf8378765f653be/xpano/algorithm/algorithm.cc#L232
• Add a a widget in the gui for selecting the value (maybe ImGui::SliderFloat): https://github.com/krupkat/xpano/blob/dee2e45635bf9ebaa88b8f098bf8378765f653be/xpano/gui/panels/sidebar.cc#L301

Stretch goal

Find / take some pictures where this effect can be demonstrated and add them to the PR.