tests: Test for common type conversion for element-wise intrinsics.

Some intrinsics have different rules for the allowed data types than
expressions:

- Vectors and matrices at the same time are not allowed, regardless of
  their dimensions. Even if they have the same number of components.

- Any combination of matrices is always allowed, even those when no
  matrix fits inside another, e.g.:
  float2x3 is compatible with float3x2, resulting in float 2x2.
  The common data type is the min on each dimension.

This is the case for max, pow, ldexp, clamp and smoothstep; which suggest that
it is the case for all intrinsics where the operation is applied element-wise.

Tests for mul() are also added as a counter-example where the operation
is not element-wise.

tests/pow.shader_test

@@ -8,3 +8,29 @@ float4 main(uniform float4 u) : sv_target
 uniform 0 float4 0.4 0.8 2.5 2.0
 draw quad
 probe all rgba (0.512, 0.101192884, 0.64, 0.25) 4
+
+
+[pixel shader todo]
+float4 main() : sv_target
+{
+    float2x3 a = {1, 2, 3, 4, 5, 6};
+    float3x2 b = {6, 5, 4, 3, 2, 1};
+
+    float2x2 r = pow(a, b);
+    return float4(r);
+}
+
+[test]
+todo draw quad
+todo probe all rgba (1.0, 32.0, 256.0, 125.0)
+
+
+[pixel shader fail todo]
+float4 main() : sv_target
+{
+    float2x2 a = {1, 2, 3, 4};
+    float4 b = {1, 2, 3, 4};
+
+    pow(a, b);
+    return 0;
+}