Small changes  to pass JSHint validation

A function in a loop and some surplus whitespace
Move loading separate from constructors

Can now instantiate the Layer and OraFile objects separate from the
constructor. This will allow to implement creating and editing next.
Exposed Layer objects and moved their loading

It is now possible to create Layer objects on their own, and their
loading is done in the OraFile. Also removed the redundant layerCount
property.
Layer indexes reversed; new add layer method

Reversed the layer indexing, so background is 0. Added a new method to
allow inserting a layer into a specific position.
Swapped order of src/dst checks in dodge

Seems to give better results on certain test files this way.
Fixes for additive and burn filters
Refactored layer drawing

Returns a canvas now instead of raw data for more versatile use.
Moved zip.js dependencies into main dir
Added custom MIME support to zip.FS
Added ability to save files

Saving with the right file type doesn't work yet though.
Updated readme