WIP on refactoring the APIs.

Moving base functionality into the `macrogl` source directory.
Instead of having WebGL and OpenGL commons in `macrogles` source
directory, we now have `macrogl` source directory.
Additional functionality is moved into `macroglex` -- the extended
API layer.
I think this is more natural.

Then, compute shader functionality (OpenGL 4.3) is moved into
`macroglex` source folder. You can see that `AttributeBuffer`'s
`computing` calls now use `Macroglex` API -- the extended API layer.

Refactoring `MeshBuffer`s to the new API model.

Review by @joelross @svreber @inmost-light.