web: shoot through the live camera

The one path where the look is chosen before the picture exists: OPEN CAMERA
grades the camera's own feed with the recipe in force, many times a second, and
the shutter hands the studio the sensor's still under that same recipe. Preview
and file differ in resolution only — the still is `takePhoto`'s own frame, not a
copy of the small preview video, with `grabFrame` and a 2d copy of the element
behind it for the browsers that ship no ImageCapture.

The renderer gains two inputs for it: `sourceImage`, a picture the caller
already decoded (re-encoding the camera's frame to JPEG only to decode it again
would cost more than the whole render), and `drawTo`, which paints the finished
picture instead of encoding it. One render is in flight at a time; a frame that
arrives during one is dropped, so a slow device shows a lower frame rate rather
than a queue of moments that have passed.

The view flashed black on a phone. Setting width/height on a canvas resets its
bitmap: measured on the preview, a resize leaves mean 0 until the next render
lands, which on this box is 0.5s and on a phone more. The buffer was sized from
every incoming frame, and a capture that renegotiates its resolution — which
Chromium does when the page is too slow to consume its frames, and this pipeline
runs ~2 fps at 720p under software GL — strobed black/picture at every switch.
The buffer is now sized on the first frame and after that only when the frame's
aspect changes: a same-aspect frame is scaled into it. Swapping a 1280x720
stream for a 640x360 one mid-view now leaves the buffer at 1280x720 with no
black frame, and 640x360 renders at 6-13 fps instead of 2.

The frames are read from a <video>, which is now IN the document (1px, behind
the black backdrop) rather than detached: Safari draws blank frames from a
detached video, which is the same black-between-pictures. It leaves the document
with the view, and the tracks are stopped, so the camera light goes out.

Probes: cam-smoke (feed painted, resolution, frame rate, a monochrome sim
reaching the live frames, shutter into the studio, close, console clean),
cam-renegotiate (no resize, no blank frame, status line on the frames),
cam-close-flip (flip returns a picture; video gone on close).

docker/frontend/src/engine/skiaShim.ts

@@ -159,6 +159,11 @@ export async function initSkia(options: InitSkiaOptions = {}): Promise<any> {
         canvasKit.MakeImageFromEncoded(
           data instanceof Uint8Array ? data : new Uint8Array(data as ArrayBuffer)
         ),
+      // An image straight off a canvas source — what the camera's own <video>
+      // is. CanvasKit draws it to a canvas and reads it back; the live view
+      // needs no JPEG round trip to hand a frame to the renderer.
+      MakeImageFromCanvasImageSource: (src: CanvasImageSource) =>
+        canvasKit.MakeImageFromCanvasImageSource(src),
       // An image from bytes rather than from a file — the tone curve's 256x1
       // lookup table (see toneCurve.ts). RN Skia takes the factory here as
       // `Image.MakeImage(info, bytes, bytesPerRow)`; CanvasKit keeps that one