Selective output geometry shader test (#1222)

Adds a geometry shader test that exercises selective primitive output
via `SV_PrimitiveID`.
Two CCW triangles tile a 2×2 render target — primitive 0 covers the
bottom half, primitive 1 the top half.
The GS emits primitive 0 unchanged and drops primitive 1 by returning
without any `Append()` calls.

Author: EmilioLaiso

test/Graphics/gs_selective_output.test

@@ -0,0 +1,106 @@
+#--- vertex.hlsl
+struct VSOutput {
+  float4 position : SV_POSITION;
+};
+
+VSOutput main(float4 position : POSITION) {
+  VSOutput o;
+  o.position = position;
+  return o;
+}
+
+#--- geometry.hlsl
+struct GSInput {
+  float4 position : SV_POSITION;
+};
+
+struct GSOutput {
+  float4 position : SV_POSITION;
+};
+
+// Selective-output GS, keyed on SV_PrimitiveID (auto-generated by the input
+// assembler). Primitive 0 (the bottom-half triangle) is emitted; primitive 1
+// (the top-half triangle) is dropped by returning without any Append() calls.
+[maxvertexcount(3)]
+void main(triangle GSInput input[3],
+          uint primID : SV_PrimitiveID,
+          inout TriangleStream<GSOutput> stream) {
+  if (primID != 0)
+    return;
+
+  GSOutput v;
+  v.position = input[0].position; stream.Append(v);
+  v.position = input[1].position; stream.Append(v);
+  v.position = input[2].position; stream.Append(v);
+}
+
+#--- pixel.hlsl
+struct PSInput {
+  float4 position : SV_POSITION;
+};
+
+float4 main(PSInput input) : SV_TARGET {
+  return float4(1.0, 0.0, 0.0, 1.0);
+}
+
+#--- pipeline.yaml
+---
+Shaders:
+  - Stage: Vertex
+    Entry: main
+  - Stage: Geometry
+    Entry: main
+  - Stage: Pixel
+    Entry: main
+Buffers:
+  # Two CCW-wound triangles tiling the 2x2 viewport: primitive 0 covers the
+  # bottom half (NDC y < 0), primitive 1 covers the top half (NDC y > 0).
+  # Both are oversized so coverage is unambiguous at every pixel center.
+  - Name: VertexData
+    Format: Float32
+    Stride: 16 # 6 vertices, 16 bytes each
+    Data: [ -3.0, -1.0, 0.0, 1.0,
+             3.0, -1.0, 0.0, 1.0,
+             0.0,  0.0, 0.0, 1.0,
+            -3.0,  1.0, 0.0, 1.0,
+             0.0,  0.0, 0.0, 1.0,
+             3.0,  1.0, 0.0, 1.0 ]
+  - Name: Output
+    Format: Float32
+    Channels: 4
+    FillSize: 64 # 2x2 @ 16 bytes per pixel
+    OutputProps:
+      Height: 2
+      Width: 2
+      Depth: 1
+Bindings:
+  VertexBuffer: VertexData
+  VertexAttributes:
+    - Format: Float32
+      Channels: 4
+      Offset: 0
+      Name: POSITION
+  RenderTarget: Output
+DescriptorSets: []
+...
+#--- end
+
+# Metal has no native geometry shader stage.
+# UNSUPPORTED: Metal
+
+# Clang's HLSL frontend does not yet support geometry shader stream types.
+# Tracked upstream: https://github.com/llvm/llvm-project/issues/136963
+# XFAIL: Clang
+
+# RUN: split-file %s %t
+# RUN: %dxc_target -T vs_6_0 -Fo %t-vertex.o %t/vertex.hlsl
+# RUN: %dxc_target -T gs_6_0 -Fo %t-geometry.o %t/geometry.hlsl
+# RUN: %dxc_target -T ps_6_0 -Fo %t-pixel.o %t/pixel.hlsl
+# RUN: %offloader %t/pipeline.yaml %t-vertex.o %t-geometry.o %t-pixel.o | FileCheck %s
+
+# Readback is top-left origin, row-major. The top row (image rows 0) was
+# covered by the dropped primitive 1, so it stays at the default clear
+# (0, 0, 0, 0). The bottom row (image row 1) was covered by the kept
+# primitive 0, so it is opaque red.
+# CHECK: Name: Output
+# CHECK: Data: [ 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 1 ]