Added check if image is already "dense" and fixed extension skipping

[giano]

Added check if image is already "dense" (in case the name already ends with “glue”+ devicePixelRatio + ‘x’), skip image with “no-dense” class and fixed extension skipping as inArray returns 0 for SVG check, that was evaluated to false and not skipped.

[gocom]

Thanks. The idea is great, but the implementation needs some work. Both security, regression, compatibility and coding style wise.

I'll will absolutely merge this once it's perfected.