Colour animations sometimes fail due to NaNs

Also seen in #4251, #4216 and probably others, colour animations sometimes don't happen due to NaNs. I've reproduced it, written a test case, found what I believe to be the bug ...

if ( fx.state == 0 ) {
    fx.start = getColor( fx.elem, attr );
    fx.end = getRGB( fx.end );
}

... and found the fix: replace the if statement with "if ( fx.start.constructor != Array)". This is because there's no guarantee that fx.state will be 0 on the first call.

The attached test case animates the backgrounds of 2500 or 10,000 divs from white to black. A small number of divs stay white. I suggest using Firefox for the test.