Only setWidth/setHeight check for NaN; setImplicitX don't even check for that.

In general, they don't even check for infinities, which looks just as wrong. F.i. an infinite high image will plain crash QtQuick because it tries to allocate a 2^30 pixels high image.

(They don't check for negative sizes as well, which IMHO doesn't make sense, but could have some meaning in the engine...)