Mock out img.src

For compatibility with flappy-birds
Revert "XMLHttpRequest: prepare responses to .html requests"

This reverts commit 89b5915f53647919522d272467235a212e7526ad.

It broke Firetext.
Mock out img.complete

Towards Cut the Rope compatibility
Support worker.postMessage before worker is loaded

For compatibility with slice-fruits
Cache set img.src

Towards compatibility with Cut the Rope