* engine: common: imagelib: add KTX2 support
Adds basic KTX2 support for a few compressed formats. KTX2 essentially
is a Vulkan-centric texture format that supports literally hundreds of
pixel formats.
For now only support for these is added:
- `VK_FORMAT_BC4_UNORM_BLOCK`
- `VK_FORMAT_BC4_SNORM_BLOCK`
- `VK_FORMAT_BC5_UNORM_BLOCK`
- `VK_FORMAT_BC5_SNORM_BLOCK`
- `VK_FORMAT_BC6H_UFLOAT_BLOCK`
- `VK_FORMAT_BC6H_SFLOAT_BLOCK`
- `VK_FORMAT_BC7_UNORM_BLOCK`
- `VK_FORMAT_BC7_SRGB_BLOCK`
Adding more formats is relatively straightforward:
- Copy format definition from `VkFormat` enum in `vulkan_core.h`
- Add a new definition into `pixformat_t` enum.
- Add format size calculation into `Image_ComputeSize()`
While we're at it, also adds a few new formats to DDS:
- BC4_UNORM -- PF_BC4_UNSIGNED
- BC4_SNORM -- PF_BC4_SIGNED
- BC5_UNORM -- PF_BC5_UNSIGNED
- BC5_SNORM -- PF_BC5_SIGNED
- BC7 is expanded into BC7_UNORM and BC7_SRGB
ref_gl and ref_soft code is updated where it made sense. But not tested
really. Support for these formats has been tested with ref_vk.
* address spaces-vs-parentheses formatting where noticed
* parenthesize sizeofs
* move ktx2.h to imagelib as img_ktx2.h; massage it a bit
* use SetBits() instead of |=
* remove stale TODO comments
