Layer blend and opacity

[jeremyfa]

It seems layer blend and opacity are not supported at the moment. Not using heaps-aseprite directly but noticed that by reading the code (and seeing opacity not taken into account in LDtk).

In my journey to implement aseprite support for Ceramic, I ported original aseprite's blend functions to Haxe, that you can find here: https://github.com/ceramic-engine/ceramic/blob/master/plugins/ase/runtime/src/ceramic/AsepriteBlendFuncs.hx

Sharing that as it could be interesting to use that in heaps-aseprite as well to improve support of layer blend/opacity (and make that work in LDtk as well?).

The actual parsing of ase data in Ceramic, also using the blend functions, can be found there: https://github.com/ceramic-engine/ceramic/blob/master/plugins/ase/runtime/src/ceramic/AsepriteParser.hx