Fixes #90739.

When Y-sorting a branch like:

 ┖╴A (y_sort_enabled = false)
    ┖╴B (y_sort_enabled = true)
       ┖╴C (y_sort_enabled = true)
          ┖╴D (y_sort_enabled = whatever)

it is flattened like:

 ┖╴A
    ┖╴B
       ┠╴B_copy
       ┠╴C
       ┖╴D

where B_copy, C, D are ordered/sorted according to their Y position within B's coordinate space.

The issue was B_copy.ysort_xform was set to inverse of B's final transform, hence B_copy would always end up being rendered at B's xform_curr as the whole final transform would be cancelled out (and because in RendererCanvasCull::_cull_canvas_item called for B_copy the B's xform_curr would be applied again). What needs to be cancelled out is only B's xform_curr.