Add proper snapping to tile polygon editor

[KoBeWi]

Closes godotengine/godot-proposals#5847

The grid does not draw, but I could add it if it's important.

[kleonc]

Made like that it's not possible to snap to subpixel values. So e.g. for odd sized tiles you won't be able to snap to the tile's center. Meaning being able to snap to half pixels is a must. So at least min/step for the spin boxes should be set to 0.5 (maybe even more granular step customization should be allowed?:thinking:).

[KoBeWi]

Odd-sized tiles are a weird use-case and I never understood why it was half-pixel in the first place, but setting step to 0.5 should be enough.

[groud]

I believe the feature is fine, but I think it takes too much space as it is and is not the best to configure IMO.
What I would do:

• Replace "grid step" in pixels by a subdivision value (defaulting to 3 or 4 I guess ?), as I don't think there are uses cases where you would need the grid with a size that's not a division of the tile size. It is also easier to setup IMO.
• Move this subdivision settings to a popup, likely accessible from the option menu dropdown.
• Add a single toggle button to enable/disable grid snapping.
• If we consider the bar is getting too large, the "snap to pixel" option might be moved inside the dropdown menu.

[ygypt]

I believe the feature is fine, but I think it takes too much space as it is and is not the best to configure IMO. What I would do:

• Replace "grid step" in pixels by a subdivision value (defaulting to 3 or 4 I guess ?), as I don't think there are uses cases where you would need the grid with a size that's not a division of the tile size. It is also easier to setup IMO.
• Move this subdivision settings to a popup, likely accessible from the option menu dropdown.
• Add a single toggle button to enable/disable grid snapping.
• If we consider the bar is getting too large, the "snap to pixel" option might be moved inside the dropdown menu.

i really like your idea of a simple one-line subdivision value, although imo putting it into a dropdown would be unnecessary. i guess its time to learn c++, i'd love to contribute to this PR :)

[nklbdev]

I created small editor plugin to help with this problem. Maybe it helps to add custom vertex snapping in future

[KoBeWi]

Updated based on feedback (including #73495 feedback)

• changed grid size to subdivisions
• the subdivision SpinBox is made smaller, so it doesn't take much space
  • it can be even smaller if we only allow 1-digit subdivisions
• snap setting and subdivision is stored in project metadata
• the grid is drawn

The original feedback was to move half-pixel to popup and make it separate from grid snap, with grid taking priority. This has a problem though: if grid is enabled, toggling half-pixel snap has no effect. So there is awkward setup where there are 4 states, but 2 are effectively the same. So instead of weird toggle/button combo, I went with 3-state popup.

godot.windows.editor.dev.x86_64_JxVow5t71w.mp4

You can either disable snap, enable half-pixel snap or enable grid snap.

[akien-mga]

Thanks!