More tightly specify HTMLMediaElement preload/readyState interaction

[domenic]

Porting from https://www.w3.org/Bugs/Public/show_bug.cgi?id=28956. Quoting the opening paragraph:

Currently the spec does not constrain the readyState values that can be observed for specific preload values. In bug 28921 we see one example of how Web pages can depend on a specific readyState-based event firing for a specific preload value, but the problem is more general than that. If a Web author wants to preload so that a specific readyState-based event fires, there is technically no way to achieve that. So I think it makes sense to define a preload value for each readyState, that guarantees the browser will keep trying to preload until that readyState is reached.

The thread contains a proposal for fixing this, and some discussions with @foolip. Related: #1932.