WIP: Improve texture loading API

[kblaschke]

Based on user feedback, the texture loading API should support a few additional features:

• Load compressed images as-is, e.g. the app just passing in the raw image file into projectM and have projectM deal with the decoding internally, just as with auto-loaded textures.
• Improve life time/ownership issue with app-created GL textures. projectM should only use, but not own/delete the texture.
• Add a way to push textures in advance, with the option to not have projectM delete them automatically. This will allow pre-loading large textures (given there's enough VRAM) and user sprite images to avoid lags later on.
• Add a callback when projectM no longer needs a user-supplied GL texture, e.g. if it's being purged from the cache. This will give apps a hint that the texture may be deleted.

In addition to these features, I've reworked the API to provide separate loading functions for compressed images, GL texture IDs and raw bitmap data, which can either be called within the texture load callback, or at any given time the app wants to push or pre-load textures. This removes the struct from the API, which would be considered immutable after releasing the new version. Adding new functions is more forward-compatible than a fixed-size struct.