lottie: can't export to png with webgl canvas

[tinyjin]

save2png currently doesn’t work in the WebGL player because the browser clears the drawing buffer after presenting a frame. While enabling preserveDrawingBuffer: true allows readback, it introduces measurable overhead by turning off driver/browser buffer optimizations. We need image export without requiring preserveDrawingBuffer.

• Use ThorVG saver and add save2png like save2gif (Need wasm binding change)
• Any bypass trick

related: thorvg/thorvg.viewer#124

[hermet]

see: thorvg/thorvg#3737