Primitive strip cut values

[kvark]

Metal has strip cut value of 0xFF..FF (depending on the index format) at all times enabled.
D3D12 allows one to choose between no value, 0xFFFF, and 0xFFFFFFFF.

It's not entirely clear how to please every backend here. Forcing the strip cut ON for everyone would require us to know the index buffer format at PSO creation for D3D12...

My proposal is to extend the Primitive type to be:

enum StripCutValue {
    Disabled,
    Ffff,
    Ffffffff,
}
enum Primitive {
   PointList,
   LineList,
   LineStrip(StripCutValue),
   TriangleList,
   TriangleStrip(StripCutValue),
   ...
}

So basically, D3D12 style with a bit of type sugar. Metal PSO creation would then return an error if the value is Disabled. An error would also be returned from using an index buffer on Metal that has a different bit width than the strip cut value.

This is mostly penalizing Metal, at the cost of not requiring extra info on PSO creation, while still supporting the strip cuts. Any ideas/corrections are welcome!

[msiglreith]

Looks nice! Especially coupling it with the Primitive type, allowing us remove This is mostly the field here and we should also got rid of this one.

[royaltm]

Currently primitive_restart property of the input_assembler is ignored in vulkan backend https://docs.rs/gfx-backend-vulkan/0.3.0/src/gfx_backend_vulkan/device.rs.html#522
in create_graphics_pipeline implementation.
I hope it's not a BUG, but a TODO?

[kvark]

@royaltm the issue is quite old, discussion predates us just settling with Vulkan API.
That code is a bug. Care to fix it with a PR?