Materials: transparency blend modes (alpha/additive) + sorting

Materials already store a RenderState (BlendMode{disabled,alpha,additive}, cull, depth) and the PBR shader supports alpha-cutoff via discard, but blended transparency is not applied — the GPU pipeline uses a zeroed (opaque) blend state and there is no draw sorting.

Scope

• Honor RenderState.blend in GpuRenderer: enable alpha/additive blend state (separate pipelines or dynamic state).
• Back-to-front sort transparent draws; keep opaque depth-prepass behavior.
• Honor RenderState.cull (back/front/none).
• Wire an alpha mode for cutoff (mask) so alpha_cutoff takes effect (shader already supports it via flags2).
• Mirror what's feasible in the software renderer.

Acceptance

• An alpha-blended material renders see-through with correct ordering; additive glows; cull mode is respected.