Add depth-only and shadow-caster pass skeletons

engine/assets/builtin/shaders/shadow-caster/shadow-caster.vs.hlsl

@@ -0,0 +1,22 @@
+// XC_BUILTIN_SHADOW_CASTER_D3D12_VS
+cbuffer PerObjectConstants : register(b0) {
+    float4x4 gProjectionMatrix;
+    float4x4 gViewMatrix;
+    float4x4 gModelMatrix;
+};
+
+struct VSInput {
+    float3 position : POSITION;
+};
+
+struct PSInput {
+    float4 position : SV_POSITION;
+};
+
+PSInput MainVS(VSInput input) {
+    PSInput output;
+    float4 positionWS = mul(gModelMatrix, float4(input.position, 1.0));
+    float4 positionVS = mul(gViewMatrix, positionWS);
+    output.position = mul(gProjectionMatrix, positionVS);
+    return output;
+}