assimp: Add assimp-disable-extensions config var (for #1537)

This can be used to disable certain extensions from being loaded via Assimp

Also makes the `get_additional_extensions()` method a bit more efficient by having it avoid string concatenation, using a temporary buffer instead

pandatool/src/assimp/loaderFileTypeAssimp.cxx

@@ -57,22 +57,49 @@ get_extension() const {
  */
 string LoaderFileTypeAssimp::
 get_additional_extensions() const {
+  // This may be called at static init time, so ensure it is constructed now.
+  static ConfigVariableString assimp_disable_extensions
+  ("assimp-disable-extensions", "",
+   PRC_DESC("A list of extensions (without preceding dot) that should not be "
+            "loaded via the Assimp loader, even if Assimp supports these "
+            "formats.  It is useful to set this for eg. gltf and glb files "
+            "to prevent them from being accidentally loaded via the Assimp "
+            "plug-in instead of via a superior plug-in like panda3d-gltf."));
+
+  bool has_disabled_exts = !assimp_disable_extensions.empty();
+
   aiString aexts;
   aiGetExtensionList(&aexts);
 
+  char *buffer = (char *)alloca(aexts.length + 2);
+  char *p = buffer;
+
   // The format is like: *.mdc;*.mdl;*.mesh.xml;*.mot
-  std::string ext;
   char *sub = strtok(aexts.data, ";");
   while (sub != nullptr) {
-    ext += sub + 2;
-    sub = strtok(nullptr, ";");
-
-    if (sub != nullptr) {
-      ext += ' ';
+    bool enabled = true;
+    if (has_disabled_exts) {
+      for (size_t i = 0; i < assimp_disable_extensions.get_num_words(); ++i) {
+        std::string disabled_ext = assimp_disable_extensions.get_word(i);
+        if (strcmp(sub + 2, disabled_ext.c_str()) == 0) {
+          enabled = false;
+          break;
+        }
+      }
+    }
+    if (enabled) {
+      *(p++) = ' ';
+      size_t len = strlen(sub + 2);
+      memcpy(p, sub + 2, len);
+      p += len;
     }
+
+    sub = strtok(nullptr, ";");
   }
 
-  return ext;
+  // Strip first space
+  ++buffer;
+  return std::string(buffer, p - buffer);
 }
 
 /**