Get the current pixel buffer of an overlay model as raw binary: width * height * bytesPerPixel bytes, row-major from the top-left, which is the exact layout `PUT /api/overlays/model/{model}/data` accepts. The model geometry is repeated in the `X-FPP-Model-Width`, `X-FPP-Model-Height` and `X-FPP-Model-BytesPerPixel` response headers.

The JSON form spends 4-6 wire bytes per payload byte; this one spends one.