KHR_texture_float_f16: Float format for material textures

[rsahlin]

The purpose of this extension is to enable use of HDR material textures by means of texture float format VK_FORMAT_R16G16B16_SFLOAT.
KTX v2 shall be used as a container, the extension is defined in a way that follows the paradigm of KHR_texture_basisu.

The intended usecase for this extension is any usecase where HDR image resources needs to be specified, for instance in an environment map (IBL) but it could be relevant for any material texture where increased range is wanted.