Generic fallback widget for every LayoutDesc element type without a
dedicated behavioral widget (chrome corners/edges, drag bars, resize grips).
Holds an ElementInfo + active-state name; draws that state's media by tiling
(UV-repeat on both S+T axes, matching ImgTex::TileCSI). DrawMode constants
documented per format spec §6 (Undefined=0, Normal=1, Overlay=2,
Alphablend=3 — no Stretch mode). Plan 1: all modes render as the same
alpha-blended tiled quad; per-mode branches deferred to Plan 2.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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