ImageElement gathererer fixups

[paulirish]

some ideas from #12065 (comment)

we could do a minor rethink of the shape and types.

• idea for imageElement.cssSizing obj width width/height props.. obj is appropriately null/undefined if skipped etc.
  • obj could include the naturalWidth bits as well and be fetchedDimensions or we keep 'em separate.
• maybe same idea for attributeWidth/etc as those don't make sense for isCss images?
• naturalWidth/naturalHeight is in a similar but not quite the same situation as cssWidth/cssHeight after core(image-elements): set 5s time budget, add _privateCssSizing #12065
• to add to the null/undefined confusion, don't forget the return value of el.getAttribute('notASetAttribute')
• '' is a bad value for attribute* for css. and also bad for html images.. should fall through to null, as getAttribute dictates.