Expose tcuTexture getChannelReadMap publicly.

- Add TextureSwizzle struct that can be used in future to implement
  swizzling in tcu::Sampler.
- Expose getChannelReadMap and WriteMap and rename them to
  getChannelReadSwizzle and WriteSwizzle.
- Use exposed channel maps in tcuTextureUtil instead of duplicating.

Change-Id: Ie4953f4d9814d7e493dbea761dc5734f7a50c57f
3 files changed