[3.x] Fix 3D Onion skinning missing

[butkeim]

Fixes #52412

3D Onion skinning was missing since #22225

AnimationPlayerEditorPlugin::forward_spatial_force_draw_over_viewport implementation was missing.

I renamed AnimationPlayerEditor::forward_canvas_force_draw_over_viewport to forward_force_draw_over_viewport for coherency.

I think I should do a PR for the master branch also, can someone confirm? (this is my first PR, sry if it's clunky)

Bugsquad edit: 3.x version of #52667.

[akien-mga]

I think I should do a PR for the master branch also, can someone confirm?

Indeed, a PR for the master branch would also be needed to make sure that this is solved in 4.0 and all future releases.

[akien-mga]

Thanks!