legacygl: preserve separate blending state

[tritao]

Summary

Preserve separate RGB and alpha blend factors in the LegacyGL renderer.

• Track whether separate blending was explicitly requested.
• Preserve separate source and destination alpha factors through lazy-element state and caches.
• Select `glBlendFuncSeparate` when separate alpha factors are active.
• Correctly distinguish ordinary blending from valid separate `ZERO/ZERO` alpha factors.
• Add focused coverage for separate alpha blending and exact GL blend-factor state.

This is a LegacyGL compatibility fix and does not affect the retained renderer architecture.

Stack

Layer 8/18, based on PR #41.