No depth and stencil format support

libktx needs to support depth and stencil formats. Currently

- `vkloader.c` and `glloader.c` would not load these formats correctly.
- `texture2.c` would allow some incorrect settings for these formats, e.g. sRGB.
- `toktx.c` makes no attempt to set things correctly when creating payloads with these formats.
- `ktx2check` does not account for the differences between these and color formats.

Start by inspecting vkloader.c, glloader.c, texture2.c, toktx.c, and ktx2check to trace how depth and stencil formats are currently handled. Done means these components load, create, and validate depth and stencil payloads without applying color-format settings such as sRGB.

Written by the indexing model from the issue text.