webgpu: Hide importExternalTexture behind flag (#5587)
webgpu: Hide importExternalTexture behind flag (#5587)

importExternalTexture API will be in heavy development after WebGPU OT.
It might break tfjs webgpu backend by accident.

Hide this API behind flag until the API is in stable mode.

This changes also fix FromPixels + HTMLVideoElement errors by drawing
HTMLVideoElement to canvas first and then using copyExternalImageToTexture

Co-authored-by: Jiajia Qin <18ef504dc2f2be7888c61ad30571995bf38d04e0@intel.com>
tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs