This addresses issue #65:
https://github.com/kanaka/noVNC/issues/65

When tightPNG encoded rects were received, any fill types were
immediately drawn to the canvas while images (PNG, JPEGs) were queued
for loading. This can cause screen corruption when things are changing
rapidly due to the misordering of fills vs images.

Also, remove the onload setting in each image on the queue and instead
decrease the tight image queue scanning interval (to 40ms or 25
scans per second).