High DPI unpacked images

[LPGhatguy]

#31 was a partial solution to the high DPI problem, and provided new a new code generation style for assets that have high DPI variants available (foo.png, foo@2x.png, etc). However, for images where packable is not set, each high DPI variant still generates code independently.

Tarmac should merge these inputs and generate one file containing all of the high DPI variants.