Speed up framebuffer emulation

screenCtx.getImageData is extremely slow on my system (Firefox on Linux amd64
with a crappy old mobile Radeon chipset). Avoiding it speeds up emulation from
unusably slow to slightly laggy.
Fix a subtle display glitch

See top left of this screenshot: http://i.imgur.com/0ZCLvKl.png
Fix checkAll() guard

The checkAll() function uses this guard to check if the emulator is ready:

    if (screen && disk.length > 0) {

But screen is a legacy DOM thing and pre-defined by the browser. This causes
a mostly harmless error if img.onload runs before window.onload.

Fix: set screen to null :)
Hide the mouse cursor over the Oberon screen
Correct mouse position in scrolled windows
Merge pull request #1 from pdewacht/javascript-fixes

Merge pull request #1: Javascript fixes