Render media in max 4:3 proportions box

[IBBoard]

My proposal to balance "I don't want tall images" with "I like media and run Corebird really wide" in Bug #465.

These changes make the visible part of the media render at max 4:3 proportions (chosen because they're old monitor proportions, fairly square and hence don't make you go "huh? what size/shape is that?", and still quite common for images based on what I saw in my timeline).

If the image is too wide then Corebird will scale the image (as it does now).

If the image is too tall then Corebird will show the middle of the image.

If the widget's bounding box is wider than the image then Corebird uses 4:3 proportions on the bounding box to calculate the max height, which will allow portrait images to show more height. (But if the image is landscape then the box won't grow too tall)