Add smooth-movement plugin

[notliad]

Summary

Adds the smooth-movement plugin to DFHack. It provides smooth visual interpolation for creatures, items, vehicles, and related overlays in the fortress viewport.

Notes

I closed the previous PR in favor of this version, which preserves the original code structure that was already reviewed by humans in the standalone repository.

Standalone repository: https://github.com/notliad/df-smooth-movement

Testing

• Built the smooth-movement plugin through the DFHack build system.
• Built and ran smooth-movement-test.
• All regression assertions passed.

[ab9rf]

A general comment as this is a pervasive issue and I do not wish to pepper the review with dozens if not hundreds of comments: there are a lot of operations in here that work on (x,y) coordinate pairs that involve effectively duplicated code. DFHack has a 2d coordinate type df::coord2d that can be used to store a coordinate pair as a unit and operate on both components in parallel. Consider using df::coord2d in lieu of separate x and y variables wherever possible as this provides clearer indication of intention as well as reducing the risk of updating one but not the other resulting in inconsistent behavior.

I've made some other specific comments but this should not be considered a full review.

[notliad]

A general comment as this is a pervasive issue and I do not wish to pepper the review with dozens if not hundreds of comments: there are a lot of operations in here that work on (x,y) coordinate pairs that involve effectively duplicated code. DFHack has a 2d coordinate type df::coord2d that can be used to store a coordinate pair as a unit and operate on both components in parallel. Consider using df::coord2d in lieu of separate x and y variables wherever possible as this provides clearer indication of intention as well as reducing the risk of updating one but not the other resulting in inconsistent behavior.

I've made some other specific comments but this should not be considered a full review.

This will be a substantial refactor. I will look into it and the others comments as well.
I'm bit worried making lots of changes cause I can't reproduce (on both of my machines) the most common bug and need someone to test it for me, this can take a day or two.

[notliad]

Camera feature was not working properly and introduced a mouse click bug so I opted to remove it.

Edit: forgot to tag @ab9rf

[ab9rf]

note: this is not a complete review - i'll make another pass after these comments are addressed

[ab9rf]

Edit: forgot to tag @ab9rf

not necessary to tag me, i'm notified on all activity on dfhack repositories