Add HQx and xBR shaders

### Description

This PR adds a new upscaling shader called [HQx](https://github.com/Armada651/hqx-shader) which is a very efficient shader with low system requirements. This shader will replace the old xBR shader as the preferred upscaling shader for low-end systems. Support for LUT textures has been added as it is required for the HQx shader.

It also updates the [xBR](https://github.com/libretro/common-shaders/tree/master/xbr) shader with its much improved multipass variants. The system requirements for this shader are now significantly higher. In return it should achieve a level of quality on par or even better than SABR at a lower cost in resources due to its multi-pass implementation.

### Screenshots

#### HQx4
![HQx](https://photos-1.dropbox.com/t/0/AACtGnedV9_krvDv5vYXODwD-zJlVFGb7fnCyeglE7cA6w/12/10853398/png/1024x768/3/1407621600/0/2/HQx.png/0zEagDqAhd2GBgMDlVPVD6wVcf_L6JPn5mkztIULnpA)

#### xBR (rounded)
![xBR](https://photos-1.dropbox.com/t/0/AABXtAs58ddph11OzLMGZwCVxqSHBk-PMssPTVyzFqxP8g/12/10853398/png/1024x768/3/1407621600/0/2/xBR.png/wPCEJuW62jABUyMyhlXu-cX17hxpOF1palWXWiK2qTE)

### To-Do
- [ ] Find out why the workaround in 5a84764 is necessary and fix it.