Include processor from pixelworks and terminate workers before creating new ones

[tschaub]

This pulls in the pixelworks dependency (which was really only depended on by this package) and terminates workers before creating new ones or when disposing of the source.

Fixes #8375.
Fixes #10856.

[werrolf]

This transfers the pixelworks assumption that a document.createElement is avaliable at import time, which is a problem for build and packaging tools.

May I suggest lazy instantiation of the canvas context, to allow canvas fallbacks to work in a browser context without breaking repackaging tools?

   hasImageData = false;
 }
 
-const context = document.createElement('canvas').getContext('2d');
+let context = null;
 
 function newImageData(data, width, height) {
   if (hasImageData) {
     return new ImageData(data, width, height);
   } else {
+    context = context || document.createElement('canvas').getContext('2d');
     var imageData = context.createImageData(width, height);
     imageData.data.set(data);
     return imageData;

See also tschaub/pixelworks#83

[tschaub]

@werrolf - Thanks for the report. Would you mind creating a new issue with this information?

[werrolf]

Try as I might, I can no longer reproduce my original problem repackaging even the original pixelworks code. I suspect a nodejs update from 8 to 10 collateral to a recent Ubuntu upgrade may have increased import lenience. With no clear steps to reproduce, I don't know how to make a good issue out of this. Sorry.