This adds the StandardMaterial TXMT from the game. We use a simple
Lambert surface shader to handle its rendering. Unfortunately since
there's no easy way in unity to make a surface shader with different
alpha modes based on the shader feature this means we need a shader
per each type of alpha mode: Opaque, CutOut and Blend.
This does lead to some duplicated code in the shaders in terms of the
properties but the body of the shader is stored in a common .cginc
file that gets pulled in with a #include