Fix flipFaceCull corrupting shared RenderState for backward-normal geometry#2915
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…ometry Material.updateRenderState() flips the face cull mode of geometry whose world scale reverses the winding order (an odd number of negative scale components), so mirrored geometry culls the correct face. It performed the flip on the RenderState returned by RenderState.copyMergedTo(), which begins "if (additionalState == null) return this;". A Material's additionalState is null until getAdditionalRenderState() is first called, so for any material that never set an additional render state, copyMergedTo() returned the shared receiver -- the technique's RenderState, or the RenderState.DEFAULT singleton -- rather than a per-material copy. flipFaceCull() then mutated that shared object in place. Because materials sharing a technique (or falling back to RenderState.DEFAULT) share that RenderState, rendering a single mirrored geometry permanently flipped the cull mode for every other geometry using it: their front faces were culled, so they vanished or rendered inside-out, and the corruption persisted across frames until another mirrored draw flipped it back. Only flip a RenderState the material owns: if copyMergedTo() handed back a shared alias, copy it into the material's private mergedRenderState first.
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Material.updateRenderState() flips the face cull mode of geometry whose world scale reverses the winding order (an odd number of negative scale components), so mirrored geometry culls the correct face. It performed the flip on the RenderState returned by RenderState.copyMergedTo(), which begins "if (additionalState == null) return this;". A Material's additionalState is null until getAdditionalRenderState() is first called, so for any material that never set an additional render state, copyMergedTo() returned the shared receiver -- the technique's RenderState, or the RenderState.DEFAULT singleton -- rather than a per-material copy. flipFaceCull() then mutated that shared object in place.
Because materials sharing a technique (or falling back to RenderState.DEFAULT) share that RenderState, rendering a single mirrored geometry permanently flipped the cull mode for every other geometry using it: their front faces were culled, so they vanished or rendered inside-out, and the corruption persisted across frames until another mirrored draw flipped it back.
Only flip a RenderState the material owns: if copyMergedTo() handed back a shared alias, copy it into the material's private mergedRenderState first.
(Created with help from AI)