[D3D12] Fake per-edge tessellation with continuous

src/xenia/gpu/d3d12/shaders/continuous_triangle.hs.hlsl

@@ -0,0 +1,35 @@
+#include "xenos_draw.hlsli"
+
+struct XeHSControlPointOutput {};
+
+struct XeHSConstantDataOutput {
+  float edges[3] : SV_TessFactor;
+  float inside : SV_InsideTessFactor;
+};
+
+XeHSConstantDataOutput XePatchConstant() {
+  XeHSConstantDataOutput output = (XeHSConstantDataOutput)0;
+  uint i;
+
+  // 1.0 is already added to the factor on the CPU, according to the images in
+  // https://www.slideshare.net/blackdevilvikas/next-generation-graphics-programming-on-xbox-360
+  // (fractional_even also requires a factor of at least 2.0).
+
+  // Don't calculate any variables for SV_TessFactor outside of this loop, or
+  // everything will be broken - FXC will add code to make it calculated only
+  // once for all 3 fork instances, but doesn't do it properly.
+  [unroll] for (i = 0; i < 3; ++i) {
+    output.edges[i] = xe_tessellation_factor_range.y;
+  }
+
+  output.inside = xe_tessellation_factor_range.y;
+
+  return output;
+}
+
+[domain("tri")]
+[partitioning("fractional_even")]
+[outputtopology("triangle_cw")]
+[outputcontrolpoints(3)]
+[patchconstantfunc("XePatchConstant")]
+void main(InputPatch<XeHSControlPointOutput, 3> input_patch) {}