[WIP] Buffers rework

[tomaka]

Close #1325

• Make VerticesSource and IndicesSource private.
• Splits Buffer between DynamicBuffer, ImmutableBuffer and PersistentBuffer.
• Modify the API of immutable and persistent buffers to be more specific.
• Split the underlying implementations in order to produce more optimized calls.
• Make the fencing strategy of persistent buffers explicit and that can be chosen by the user.
• Rework index buffers.
• Rework vertex buffers.
• Rework buffer textures.
• Rework multidraw indirect buffers.
• Rework uniform buffers.
• Rework pixel buffers.
• Remove all BufferAny types.
• Allow converting a PersistentBuffer to a SyncPersistentBuffer (Sync but not Send) for multithreaded reading and writing.

This also changes how the specializations work.
Instead of being full wrappers, types such as VertexBuffer or PixelBuffer are not templated. For example instead of IndexBuffer<T> you will now have IndexStorage<Buffer<[T]>>, so that it can be adapted to all other underlying buffer types. Old type names will still be provided as aliases, so this shouldn't break anything.

This is a breaking change because:

• VerticesSource and IndicesSource are now private.
• persistent, immutable, empty_persistent, empty_immutable are removed in favor of new and empty on the new types.

[nicokoch]

Are you still working on this?

[tomaka]

No :-/

As I said I'm thinking of reorienting glium towards a more OpenGL 2-OpenGL 3-based experience.
For example I've been thinking of removing persistant-mapped buffers entirely.