cute_aseprite.h - Aseprite v1.3 property map support

[RandyGaul]

https://github.com/aseprite/aseprite/blob/main/docs/ase-file-specs.md#user-data-chunk-0x2020

Aseprite v1.3 has property map user data chunks. The parser cannot currently handle these. Some research needs to happen to figure out how these maps are used/what they're for. Unfortunately it looks pretty complex, with some arrays and nesting... For the time being this feature isn't supported.