fix: keep support interface intact with variable layer heights

[zackaree-shen]

Summary

Fixes support interface being fragmented (only ~21% area remaining) when variable layer heights are enabled on a model with a large overhang.

Root cause

trim_support_layers_by_object trims top contact layers using object layer lslices. With variable layer heights, the synchronised gap (sync_gap_with_object_layer) is shorter than the configured gap, so the object layer directly supported by the contact layer falls inside the trimming window. That layer's lslices cover the overhang region itself and trimmed ~80% of the contact area, leaving only fragmented support interface mixed with base support.

Fix

Skip the object layer directly supported by a top contact layer (i == support_layer.idx_object_layer_above) when collecting trimming polygons. This keeps the contact layer intact; fixed layer height behavior is unchanged (verified via debug logs: interface area 1.59e15 covers base fully, base_after = 0).

Verification

• Variable layer height: support interface restored to full coverage
• Fixed layer height: no regression (interface coverage 100%, no base residue)

Impact

• src/libslic3r/Support/SupportMaterial.cpp (12 lines added)
• Applies to normal (non-tree) supports; tree support path is unaffected (its trim function operates on print support layers without layer_type/idx members).