Tile artefacts in Chrome (when OffscreenCanvas used)

[Vitaly Puzrin]

Ref: #219 (comment)

In Chrome test image shows defects on tile's borders, when webworked used.

Workaround:

pica/lib/worker.js

Line 33 in 384784f

 if (returnBitmap) { 

If force return typed array, defect disappears.

[John Costella]

A test image that showed the artifacts is here.