* Create a cache module to query and load cached files without having to load or
download the original file, as well as comparing changed time and size.
* Change `loadUri` to allow fetching the modification time and size of a file.
* Change `onload` of `loadable` to have a single `FetchResult` objects instead
of many arguments (ok, err, data, time, size).
* Removed Pixie support.
* Added a buffer copy to simplify potentially problematic code.
* Add cache_key to newTexture when loading from buffer.
* For packed images, use the name of the blend file and the image as cache_key.
* Procs `loadFileFromPointersLen`, `getDimensionsFormat` and related graphics
API-agnostic functions have been moved into their own module `texture_decode`.
* New directory `gpu_formats` added, where we will put all modules related to
decoding, optimizing and caching meshes and textures (independently of the
graphics API being used) in a separate thread or threads.
