Trade: print layer index in MaterialData duplicate attribute assert.

With really huge materials it's kinda useless to not know which layer
the error happened in -- and usually it's exactly because the layer
indices were specified wrong.

src/Magnum/Trade/MaterialData.cpp

@@ -233,7 +233,7 @@ MaterialData::MaterialData(const MaterialTypes types, Containers::Array<Material
         if(end - begin > 1) for(std::size_t j = begin + 1; j != end; ++j) {
             if(_data[j - 1].name() < _data[j].name()) continue;
 
-            std::sort(_data + begin, _data + end, [](const MaterialAttributeData& a, const MaterialAttributeData& b) {
+            std::sort(_data + begin, _data + end, [i](const MaterialAttributeData& a, const MaterialAttributeData& b) {
                 /* Need to check here (instead of in the outer for loop) as we
                    exit the loop right after the sort and thus duplicates that
                    occur after the first non-sorted pair wouldn't get detected.
@@ -244,7 +244,7 @@ MaterialData::MaterialData(const MaterialTypes types, Containers::Array<Material
                    which is undesirable). Apparently libc++ is responsible for
                    such atrocities. */
                 CORRADE_ASSERT(&a == &b || a.name() != b.name(),
-                    "Trade::MaterialData: duplicate attribute" << a.name(), false);
+                    "Trade::MaterialData: duplicate attribute" << a.name() << "in layer" << i, false);
                 return a.name() < b.name();
             });
             break;

src/Magnum/Trade/Test/MaterialDataTest.cpp

@@ -978,6 +978,11 @@ void MaterialDataTest::constructDuplicateAttribute() {
     #endif
 
     Containers::Array<MaterialAttributeData> attributes{InPlaceInit, {
+        /* This attribute is in the first layer, so it should not be reported
+           as duplicated */
+        {MaterialAttribute::DoubleSided, true},
+
+        /* Second layer is empty, this is the third layer (index 2) */
         {MaterialAttribute::DoubleSided, true},
         {MaterialAttribute::DiffuseTextureCoordinates, 5u},
         {"highlightColor", 0x335566ff_rgbaf},
@@ -987,16 +992,16 @@ void MaterialDataTest::constructDuplicateAttribute() {
 
     /* Testing that it asserts in all input permutations */
     for(std::size_t i = 0; i != testCaseRepeatId(); ++i)
-        std::next_permutation(attributes.begin(), attributes.end(), [](const MaterialAttributeData& a, const MaterialAttributeData& b) {
+        std::next_permutation(attributes.begin() + 1, attributes.end(), [](const MaterialAttributeData& a, const MaterialAttributeData& b) {
             return a.name() < b.name();
         });
 
     std::ostringstream out;
     Error redirectError{&out};
-    MaterialData data{{}, std::move(attributes)};
+    MaterialData data{{}, std::move(attributes), Containers::array<UnsignedInt>({1, 1, 6})};
     /* Because with graceful asserts it doesn't exit on error, the assertion
        might get printed multiple times */
-    CORRADE_COMPARE(Utility::String::partition(out.str(), '\n')[0], "Trade::MaterialData: duplicate attribute DiffuseTextureCoordinates");
+    CORRADE_COMPARE(Utility::String::partition(out.str(), '\n')[0], "Trade::MaterialData: duplicate attribute DiffuseTextureCoordinates in layer 2");
 }
 
 void MaterialDataTest::constructFromImmutableSortedArray() {