Intrinsic width is bogus

[zcorpan]

"The intrinsic width of el is the result of finding the effective size from source list."
http://picture.responsiveimages.org/#select-an-image-source

I just realized that this is bogus. For starters, if the sizes attribute is absent, the intrinsic width will be set to 300px every time the UA decides to start a new image load. Even if the sizes attribute is present, the values in it are supposed to be approximate, not appropriate for setting the intrinsic width of the image.

If anything we should use the width descriptor and divide by the density. Then calculate the intrinsic height from the aspectratio attribute (#85). If we don't know both the width and height, maybe we shouldn't update either of them when selecting an image source (since it would cause the old image to be stretched).