Macros 1.1 custom derives

[kvark]

This PR introduces custom derive macros VertexData and ConstantBuffer. It is an alternative to macro-generated gfx_vertex_struct and gfx_constant_struct, aimed at better flexibility.

Current limitations:

• the shader name has to match the field name
• gfx namespace has to be visible

New features:

• you control the privacy of the struct and its fields
• you can derive more traits (like Eq) at will
• less magic is going on with your code

Published crate: https://crates.io/crates/gfx_macros/0.2.0

Example:

#[derive(VertexData)]
struct Vertex {
    a_pos: [u8; 4],
    a_tex_coord: [f32; 2],
}

[kvark]

r? anyone

[Bastacyclop]

is the Gfx prefix necessary ? (I hope there is a way to rename a macro on the user side if he has clashes)

[Bastacyclop]

We are not really deriving GfxStruct, I think rustc already wraps the derive panics inside a pretty message with the derive name and location.

[kvark]

Nicely spotted! Fixed in #1207

[kvark]

Thanks @Bastacyclop ! Fixed now, merging in an hour.

[ebkalderon]

This is fantastic! ❤️ I think this also means that my previous issues with #![deny(missing_docs)] is no longer a problem, correct?