Implement texture filtering

[bvssvni]

Depends on PistonDevelopers/texture#32.

[bvssvni]

Seems SamplerInfo is read-only https://gfx-rs.github.io/gfx/gfx/device/handle/struct.Sampler.html.

Could be related to gfx-rs/gfx#311.

[kvark]

You'd need to create a different Sampler for a different filtering mode, so SamplerInfo is immutable by design. For binding, a TextureParam = (handle::Texture, Option<handle::Sampler>), so you can mix and match them at run time.

[nanotech]

This is partially fixed by #309 and PistonDevelopers/gfx_texture/pull/89.