Metal: Fix crash when compiling compositor shaders.

Although viewport compositor isn't supported yet on Apple deviced the
shaderlibs are compiled. The compositor shaders uses mat3 constructor
with a single vec3 and 6 floats. This constructor wasn't defined in
metal so it failed the compilation.

This patch adds the override to mat3.

source/blender/gpu/shaders/metal/mtl_shader_defines.msl

@@ -1236,6 +1236,11 @@ mat3 MAT3x3(
 {
   return mat3(vec3(a1, a2, a3), vec3(b1, b2, b3), vec3(c1, c2, c3));
 }
+mat3 MAT3x3(
+    vec3 a, float b1, float b2, float b3, float c1, float c2, float c3)
+{
+  return mat3(a, vec3(b1, b2, b3), vec3(c1, c2, c3));
+}
 mat3 MAT3x3(float f)
 {
   return mat3(f);