d3d12: emulate missing vertex-formats
OpenGL 3.0 also requires support for 8 and 16 bit signed and unsigned RGB vertex formats. So let's emulate these as well.
The lack of these lead to asserts in the spec@!opengl 3.0@gl-3.0-vertexattribipointer
-piglit.
This also includes a bugfix for the shader-key logic that would have made all subsequent test-cases fail after we triggered emulation for the first time.
Edited by Erik Faye-Lund