Added --gpu-index to forwardable_cli_arguments

when the gpu index is specified through the CLI, that setting will
be inherited by both the editor (if started through project manager)
and instances of the game started through the editor

Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>

main/main.cpp

@@ -856,6 +856,15 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 				forwardable_cli_arguments[CLI_SCOPE_TOOL].push_back(I->next()->get());
 			}
 		}
+		// If gpu is specified, both editor and debug instances started from editor will inherit.
+		if (I->get() == "--gpu-index") {
+			if (I->next()) {
+				forwardable_cli_arguments[CLI_SCOPE_TOOL].push_back(I->get());
+				forwardable_cli_arguments[CLI_SCOPE_TOOL].push_back(I->next()->get());
+				forwardable_cli_arguments[CLI_SCOPE_PROJECT].push_back(I->get());
+				forwardable_cli_arguments[CLI_SCOPE_PROJECT].push_back(I->next()->get());
+			}
+		}
 #endif
 
 		if (adding_user_args) {