Generate fewer media queries

[dawaltconley]

Currently, the generateMediaQueries function outputs a media query for every device, regardless of whether that device reuses the same image as another. This can be optimized by removing media queries that don't affect the image selection.

It could also be optimized by bundling the media queries, as with postcss-sort-media-queries. This only affects CSS output though and wouldn't work with picture elements, which are currently unimplemented but would use the same function.

Generating fewer queries will actually make them harder to bundle, but it is probably worth doing both.