Add more support for cgmath and nalgebra types

[Rua]

Changelog:

### Additions
Better cgmath and nalgebra support, enabled by the `cgmath` or `nalgebra` features:
- `VertexMember` is now implemented for cgmath `Vector`s and `Point`s.
- `type_for_format_cgmath` and `type_for_format_nalgebra` macros, next to the existing `type_for_format` macro.
- Vulkano-shaders: `shader_cgmath` and `shader_nalgebra` macros, next to the existing `shader` macro.

Closes #1926.