Keep the pixel data when capturing a frame from an image element

[mattdawkins]

• imageElementToRgba spread an ImageData; its data, width and height are prototype getters, so the result had no pixels in Firefox (Chromium happened to keep data)
• Symptoms on image sequences in Firefox: stereo fast-mode line transfer fails with "can't access property 0, e is undefined", and the browser SAM click in Add SAM2/SAM3 browser segmentation and stereo auto-population #1672 fails with "this.data is undefined"
• The same change is on the Add SAM2/SAM3 browser segmentation and stereo auto-population #1672 branch

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