[gnilobaiter]

Clear stale Skyboxify sky layers (optional compat fix)

Refs #108, related to #90.

This is an optional compat workaround. Feel free to drop it. If it looks useful, it could go into a future release as a fix for a mod whose source is closed, so it can't be fixed upstream.

The problem

With RenderScale + Skyboxify + a custom sky pack (Dramatic Skys), there is a large frametime spike (~200-600 ms in my logs) when leaving water after ~10-20 seconds underwater. The spike gets worse as the render scale goes up (none at 100%, small at 105%, bigger at 150%, worst at 200%).

Cause

Skyboxify adds its sky layers to an internal queue (FeatureRenderer.submits) every frame during extraction, but only clears it in endFrame, which runs when the vanilla sky pass renders. Vanilla skips the sky pass in some situations (it happens underwater), so the queue is never cleared and grows by a few layers per frame. When the sky pass runs again, every queued layer is drawn in one frame.

I logged the queue size while underwater: it grew linearly, 180 -> 7260 in ~6 seconds, and dropped back to 4 as soon as I surfaced. In that same frame Dynamic Transforms UBO resized from 32 up to 2048+ (16384 and 32768 after longer dives) and the frame took 200+ ms. All of these are full-screen layers, so the cost scales with the render target size, which is why it shows up with RenderScale.

What this changes

• New SkyboxifyCompat: after renderLevel finishes, it calls the public FeatureRenderer.clear() on Skyboxify's queue.
• MixinGameRenderer.handBack calls it once per frame.

In a normal frame the queue is already empty after endFrame, so this is a no-op. When the sky pass was skipped, it drops the layers that would otherwise pile up.

Implementation notes

• Everything is reflective, so Skyboxify stays an optional dependency. If the classes/fields are missing (Skyboxify not installed, or a different layout) it silently does nothing. If a reflective call throws, it logs a warning once and disables itself.
• It logs one info line when Skyboxify is detected.
• The proper fix belongs in Skyboxify: clear the queue even when the sky pass is skipped. This just works around it on our side.
• Field/class names are taken from Skyboxify 3.4 for 26.3 (skyboxify$skyFeatureRenderer on SkyRenderer, FeatureRenderer#clear).

Testing

• Built 26.3-fabric only. I have not built the other version targets, but the change is pure reflection plus one call, so it shouldn't affect them.
• In game on 26.3 Fabric (Skyboxify 3.4, Sodium, Dramatic Skys, scale 200%): the spike on leaving water is gone, and Dynamic Transforms UBO no longer grows after diving.
• Smaller frametime spikes while chunks load still happen. They don't look related to this change or to Skyboxify.