Export opacity is 3DS

closes https://github.com/assimp/assimp/issues/3291

code/AssetLib/3DS/3DSExporter.cpp

@@ -290,12 +290,17 @@ void Discreet3DSExporter::WriteMaterials() {
             ChunkWriter curChunk(writer, Discreet3DS::CHUNK_MAT_SPECULAR);
             WriteColor(color);
         }
-
+                
         if (mat.Get(AI_MATKEY_COLOR_AMBIENT, color) == AI_SUCCESS) {
             ChunkWriter curChunk(writer, Discreet3DS::CHUNK_MAT_AMBIENT);
             WriteColor(color);
         }
 
+        if (mat.Get(AI_MATKEY_OPACITY, f) == AI_SUCCESS) {
+            ChunkWriter chunk(writer, Discreet3DS::CHUNK_MAT_TRANSPARENCY);
+            WritePercentChunk(1.0f - f);
+        }
+
         if (mat.Get(AI_MATKEY_COLOR_EMISSIVE, color) == AI_SUCCESS) {
             ChunkWriter curChunk(writer, Discreet3DS::CHUNK_MAT_SELF_ILLUM);
             WriteColor(color);