use img width and height attributes as gm filtering input

[Munter]

Setting width and height of an image, thus sometime maybe resizing it, could potentially be used as dimension input information for the graphicsmagick resize filter.

[papandreou]

So if you have <img width=100 height=200 src=foo.png>, where the width and height attributes will cause foo.png to be scaled down by the browser, foo.png should be resized during the build?

Sounds reasonable, although the code that does this should probably apply the resize only if the <img> is foo.png's only incoming relation, or if every incoming relation is an <img> that scales it to those exact dimensions.