No depth and stencil format support

[MarkCallow]

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.

[MarkCallow]

texture2.c has been fixed. The new ktx validate and ktx create correctly support depth & stencil. The legacy tools will not be fixed. That leaves vkloader.c and glloader.c to be fixed.