Add high DPI modifier to resize operations

[Munter]

When generating multiple versions of the same image for high DPI device responsive images it would be nice to be able to just add @2x, @4x, @1.4x or whatever other crazy high DPI resolutions that exist out there. Currently this resizing math has to be done by the developer, which seems a bit tedious.

If we can find some research about lossy image quality reduction for high dpi resolutions we might also have an @4x expand to set a different jpeg quality or something similar

[papandreou]

Great idea!