Bindless textures

[tomaka]

Summary of the changes:

• The buffer types no longer require their content to Send or 'static, except for buffers types which don't have type information.
• Add the TextureHandle<'a> type that can be put into uniform buffers/SSBOs. Vertex buffers and regular uniforms are not yet supported.
• A texture can be turned into a ResidentTexture by calling resident or resident_if_supported.
• A TextureHandle<'a> can be built from a &'a ResidentTexture and can't outlive the resident texture.

The implementation should be entirely safe.

Future work:

• Add bindless textures support for vertex buffers and regular uniforms.
• Support using a sampler.
• Image load/store.
• Consider strong typing, with ResidentTexture2dArray and Texture2dArrayHandle for example.

Close #243
cc #878

[tomaka]

Someone made a good remark on IRC. If ResidentTexture takes ownership of the texture, that means we can't use the same texture with multiple different samplers. This is bad.

[tomaka]

There are two solutions:

• Either make ResidentTexture take a reference to a texture instead of taking ownership of it.
• Or make ResidentTexture hold multiple handles, one per sampler. This would require changing TextureHandle::set(&ResidentTexture) to TextureHandle::set(&ResidentTexture, &SamplerBehavior).

I think that the second option is much easier to use for the user.