Mesh API overhaul (#599 Followup) #756

So #599 got merged, but there's more to it:

• Add API to customize attribute data befor passing to buffer to allow:
  Float -> Half/NormU16/... conversion
  Attribute Packing (is that even still a thing?)
  normal compression
  (custom initialization of undefinied attributes ?)
• Allow for multiple attribute buffers.
• Replace buffer names with hashes for tighter structs?
• Allow for VertexAttributeValues to be unloaded after beeing send to GPU to save CPU memory. Mesh: added static and dynamic mesh for less runtime memory usage #1782
• Allocated one big fallback buffer instead of speperate ones for each mesh. Removed vertex fallback buffer #870
• Provide example. (see Example for custom mesh attributes #757)
• Quick accsess to common attributes, eg. Mesh::set_positions(positions: Vec<[f32; 3]>)?