Various fixes

[Eearslya]

Just a handful of fixes I put together while I was implementing fastgltf.

• Adds parseAnimations() to parseAll().
• Adds name field to Animation.
• Fixes false error result when AnimationSampler.interpolation is missing.
• Material textures now stay at std::nullopt when not specified in glTF.
• Fix unaligned memory access in Base64 decoding.