[D3D12] Fake per-edge tessellation with continuous

src/xenia/gpu/d3d12/shaders/tessellation_quad.vs.hlsl

@@ -0,0 +1,5 @@
+float4 main(uint xe_vertex_id : SV_VertexID) : SV_Position {
+  uint2 position = (xe_vertex_id >> uint2(1u, 0u)) & 1u;
+  position.y ^= position.x;
+  return float4(float2(position), 0.0, 1.0);
+}