Require explicit VertexInputState creation

[Rua]

Changelog:

### Breaking changes
Changes to shaders and pipelines:
- `GraphicsPipelineBuilder::vertex_input_state` now requires a `VertexInputState` object directly, instead of a `VertexDefinition`. The `VertexDefinition` trait can be used to create the object.

This makes things a bit more explicit, and also eliminates the last type parameter from the builder.

After this, I intend to give the pipeline layout the same treatment, but while still allowing the user to auto-create it if they really want to.