Optimizations

[martinRenou]

I feel like there could be some optimizations concerning the image update.

1. We should draw binary data directly on the canvas using:

context.putImageData(binary_data);

instead of creating a Blob that is png encoded. That would also allow the back-end to not waste time encoding into png (if that is possible).

2. I feel like the "diff" computation should actually be slower than sending the full image, as it needs an extra computation of "which pixels actually needs a redraw?": https://github.com/matplotlib/matplotlib/blob/5fc44eea64a57ab1c6d236cf7c71906c9d40e6f6/lib/matplotlib/backends/backend_webagg_core.py#L190-L195.
   Plus, we are still sending a full image. Maybe the png encoding does some compression if lots of pixels are transparent? If that is not the case, we are still sending the same amount of data.

[tacaswell]

iirc @mdboom did some bench marking on this to get to the current set up.

png will do run-length encoding so if you have many identical pixels in a row, you will get pretty extreme compression.