HLSL: matrix and vector truncations for m*v, v*m, m*m

HLSL truncates the vector, or one of the two matrix dimensions if there is a
dimensional mismatch in m*v, v*m, or m*m.

This PR adds that ability.  Conversion constructors are added as required.

Test/hlsl.mul-truncate.frag

@@ -0,0 +1,37 @@
+
+// Test v*v, v*m, m*v, and m*m argument clamping.
+
+cbuffer Matrix
+{
+    float4x4  m44;
+    float4x3  m43;
+    float3x4  m34;
+    float2x4  m24;
+    float4x2  m42;
+    float4    v4;
+    float3    v3;
+    float2    v2;
+}
+
+float4 main() : SV_Target0
+{
+    // v*v:
+    float  r00 = mul(v2, v3);  // float = float2*float3; // clamp to float2 dot product
+    float  r01 = mul(v4, v2);  // float = float4*float2; // clamp to float2 dot product
+
+    // v*m
+    float4 r10 = mul(v3, m44); // float4 = float3 * float4x4;  // clamp mat to float3x4;
+    float4 r11 = mul(v4, m34); // truncate vector to vec3
+
+    // m*v
+    float4 r20 = mul(m44, v3); // float4 = float4x4 * float3;  // clamp mat to float4x3;
+    float4 r21 = mul(m43, v4); // truncate vector to vec3
+
+    // // m*m
+    // float2x3 r30 = mul(m24, m33);  // float2x3 = float2x4 * float3x3;
+    // float3x4 r31 = mul(m33, m24);  // float3x4 = float3x3 * float2x4;
+    // float3x2 r32 = mul(m33, m42);  // float3x2 = float3x3 * float4x2;
+    // float4x3 r33 = mul(m42, m33);  // float4x3 = float4x2 * float3x3;
+
+    return r10 + r11 + r20 + r21 + r00 + r01; // + r30[0].x + r31[0] + r32[0].x + transpose(r33)[0];
+}