demos/triangle: Add a demo program.

demos/triangle.hlsl

@@ -0,0 +1,20 @@
+struct ps_in
+{
+    float4 position : SV_POSITION;
+    float4 colour : COLOR;
+};
+
+struct ps_in vs_main(float4 position : POSITION, float4 colour : COLOR)
+{
+    struct ps_in o;
+
+    o.position = position;
+    o.colour = colour;
+
+    return o;
+}
+
+float4 ps_main(struct ps_in i) : SV_TARGET
+{
+    return i.colour;
+}