fix: panes keep their WebGL renderer, and give the context back when they lose it

[ba2slk]

Two things a scroll or a switch could leave on screen. First, a pane already in view turned white with a sad-face icon in its corner: Chromium's drawing for a canvas whose WebGL context is lost. WebglAddon.dispose() removes the canvas but leaves the context alive until garbage collection, and Chromium counts it against its per-page cap until then, so panes leaving the screen left dead contexts behind, the browser saw more than sixteen, and it force-lost the oldest live one until the garbage was collected. Detaching now loses the context explicitly through WEBGL_lose_context; eight switches between a fourteen-pane and a nine-pane session, screencast at device pixels, went from 13 white frames of 87 to 0 of 83. Second, a long scroll or a held Alt+→ across a wide canvas ended on one blank frame in the panes that arrived: they had given their renderer back on leaving the visible band and took a fresh one on return, which paints a frame late. The budget now keeps the renderer of a pane out of view while there is room under the cap and gives idle ones up least recently seen first only when a visible pane needs the slot; four end-to-end round trips over a 5600px canvas went from 40 renderer attaches to 0 after first sight. The self-check counts WebGL canvases specifically and asserts that no canvas still in the document loses its context during the held-key burst. Details are in docs/engineering-notes.md.

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