Reset scene when vello/vello_hybrid window renderers skip a frame

[nicoburns]

Summary

Fixes #98.

VelloWindowRenderer::render and VelloHybridWindowRenderer::render only called self.scene.reset() at the end of a successful frame. When target_texture_view() failed (e.g. occluded/off-screen window) or maybe_blit_and_present() errored, they returned early after draw_fn had already encoded into the scene, so the next frame appended on top of it. Over many skipped frames the scene grows unbounded (in vello_hybrid this eventually panics with "alpha_idx too large").

Both early-return paths after draw_fn now reset the scene too.

The other window renderers (skia, softbuffer, pixels) aren't affected: their early returns happen before drawing, or they don't keep a scene between frames.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/c50e2a3e1b7b4eb9af41e07d8ffb3896
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/c50e2a3e1b7b4eb9af41e07d8ffb3896?variant=devin-insiders
Requested by: @nicoburns

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