Implement the `%command%` placeholder in the Main Run Args setting

This can be used to tell Godot to run an executable that will run Godot
rather than running Godot directly. This is useful to make Godot start
on the dedicated GPU when using a NVIDIA Optimus setup on Linux:
`prime-run %command%`

The `editor/run/main_run_args` setting declaration was moved to make it
visible in the ProjectSettings documentation.

core/config/project_settings.cpp

@@ -1104,6 +1104,8 @@ ProjectSettings::ProjectSettings() {
 	}
 	extensions.push_back("shader");
 
+	GLOBAL_DEF("editor/run/main_run_args", "");
+
 	GLOBAL_DEF("editor/script/search_in_file_extensions", extensions);
 	custom_prop_info["editor/script/search_in_file_extensions"] = PropertyInfo(Variant::PACKED_STRING_ARRAY, "editor/script/search_in_file_extensions");
 

doc/classes/OS.xml

@@ -143,7 +143,7 @@
 				Returns the command-line arguments passed to the engine.
 				Command-line arguments can be written in any form, including both [code]--key value[/code] and [code]--key=value[/code] forms so they can be properly parsed, as long as custom command-line arguments do not conflict with engine arguments.
 				You can also incorporate environment variables using the [method get_environment] method.
-				You can set [code]editor/main_run_args[/code] in the Project Settings to define command-line arguments to be passed by the editor when running the project.
+				You can set [member ProjectSettings.editor/run/main_run_args] to define command-line arguments to be passed by the editor when running the project.
 				Here's a minimal example on how to parse command-line arguments into a dictionary using the [code]--key=value[/code] form for arguments:
 				[codeblocks]
 				[gdscript]

doc/classes/ProjectSettings.xml

@@ -542,6 +542,14 @@
 		<member name="editor/node_naming/name_num_separator" type="int" setter="" getter="" default="0">
 			What to use to separate node name from number. This is mostly an editor setting.
 		</member>
+		<member name="editor/run/main_run_args" type="String" setter="" getter="" default="&quot;&quot;">
+			The command-line arguments to append to Godot's own command line when running the project. This doesn't affect the editor itself.
+			It is possible to make another executable run Godot by using the [code]%command%[/code] placeholder. The placeholder will be replaced with Godot's own command line. Program-specific arguments should be placed [i]before[/i] the placeholder, whereas Godot-specific arguments should be placed [i]after[/i] the placeholder.
+			For example, this can be used to force the project to run on the dedicated GPU in a NVIDIA Optimus system on Linux:
+			[codeblock]
+			prime-run %command%
+			[/codeblock]
+		</member>
 		<member name="editor/script/search_in_file_extensions" type="PackedStringArray" setter="" getter="" default="PackedStringArray( &quot;gd&quot;, &quot;shader&quot; )">
 			Text-based file extensions to include in the script editor's "Find in Files" feature. You can add e.g. [code]tscn[/code] if you wish to also parse your scene files, especially if you use built-in scripts which are serialized in the scene files.
 		</member>

editor/editor_node.cpp

@@ -5877,8 +5877,6 @@ EditorNode::EditorNode() {
 
 	register_exporters();
 
-	GLOBAL_DEF("editor/run/main_run_args", "");
-
 	ClassDB::set_class_enabled("RootMotionView", true);
 
 	//defs here, use EDITOR_GET in logic

editor/editor_run.cpp

@@ -183,15 +183,50 @@ Error EditorRun::run(const String &p_scene, const String &p_custom_args, const L
 		args.push_back(p_scene);
 	}
 
+	String exec = OS::get_singleton()->get_executable_path();
+
 	if (p_custom_args != "") {
-		Vector<String> cargs = p_custom_args.split(" ", false);
-		for (int i = 0; i < cargs.size(); i++) {
-			args.push_back(cargs[i].replace(" ", "%20"));
+		// Allow the user to specify a command to run, similar to Steam's launch options.
+		// In this case, Godot will no longer be run directly; it's up to the underlying command
+		// to run it. For instance, this can be used on Linux to force a running project
+		// to use Optimus using `prime-run` or similar.
+		// Example: `prime-run %command% --time-scale 0.5`
+		const int placeholder_pos = p_custom_args.find("%command%");
+
+		Vector<String> custom_args;
+
+		if (placeholder_pos != -1) {
+			// Prepend executable-specific custom arguments.
+			// If nothing is placed before `%command%`, behave as if no placeholder was specified.
+			Vector<String> exec_args = p_custom_args.substr(0, placeholder_pos).split(" ", false);
+			if (exec_args.size() >= 1) {
+				exec = exec_args[0];
+				exec_args.remove(0);
+
+				// Append the Godot executable name before we append executable arguments
+				// (since the order is reversed when using `push_front()`).
+				args.push_front(OS::get_singleton()->get_executable_path());
+			}
+
+			for (int i = exec_args.size() - 1; i >= 0; i--) {
+				// Iterate backwards as we're pushing items in the reverse order.
+				args.push_front(exec_args[i].replace(" ", "%20"));
+			}
+
+			// Append Godot-specific custom arguments.
+			custom_args = p_custom_args.substr(placeholder_pos + String("%command%").size()).split(" ", false);
+			for (int i = 0; i < custom_args.size(); i++) {
+				args.push_back(custom_args[i].replace(" ", "%20"));
+			}
+		} else {
+			// Append Godot-specific custom arguments.
+			custom_args = p_custom_args.split(" ", false);
+			for (int i = 0; i < custom_args.size(); i++) {
+				args.push_back(custom_args[i].replace(" ", "%20"));
+			}
 		}
 	}
 
-	String exec = OS::get_singleton()->get_executable_path();
-
 	printf("Running: %s", exec.utf8().get_data());
 	for (List<String>::Element *E = args.front(); E; E = E->next()) {
 		printf(" %s", E->get().utf8().get_data());