svga: fix invalid component access of domain location

Tesscoord is declared as vec3 in the incoming shader but the z component of a tesscoord should only be referenced in the domain shader if the tessellator domain is of triangle type.

Fixes vmx crash running GFXBench-Tessellation with MTL Renderer.

Reviewed-by: Martin Krastev krastevm@vmware.com Reviewed-by: Min-Yu Huang min-yuhuang@vmware.com