Add a --max-fps command-line argument to set a FPS limit

This allows limiting framerate on any project, which is useful to
reduce power usage and latency with certain setups (such as VRR displays).

This is particularly useful in projects that do not expose a setting to change
the FPS limit. While external FPS limiters can be used, they can be cumbersome
to set up and result in increased input lag compared to a built-in FPS limiter.

doc/classes/ProjectSettings.xml

@@ -345,6 +345,7 @@
 			If [member display/window/vsync/vsync_mode] is [code]Enabled[/code], on monitors with variable refresh rate enabled (G-Sync/FreeSync), using a FPS limit a few frames lower than the monitor's refresh rate will [url=https://blurbusters.com/howto-low-lag-vsync-on/]reduce input lag while avoiding tearing[/url].
 			If [member display/window/vsync/vsync_mode] is [code]Disabled[/code], limiting the FPS to a high value that can be consistently reached on the system can reduce input lag compared to an uncapped framerate. Since this works by ensuring the GPU load is lower than 100%, this latency reduction is only effective in GPU-bottlenecked scenarios, not CPU-bottlenecked scenarios.
 			See also [member physics/common/physics_ticks_per_second].
+			This setting can be overridden using the [code]--max-fps <fps;>[/code] command line argument (including with a value of [code]0[/code] for unlimited framerate).
 			[b]Note:[/b] This property is only read when the project starts. To change the rendering FPS cap at runtime, set [member Engine.max_fps] instead.
 		</member>
 		<member name="audio/buses/channel_disable_threshold_db" type="float" setter="" getter="" default="-60.0">

main/main.cpp

@@ -210,6 +210,7 @@ static bool debug_paths = false;
 static bool debug_navigation = false;
 static bool debug_avoidance = false;
 #endif
+static int max_fps = -1;
 static int frame_delay = 0;
 static bool disable_render_loop = false;
 static int fixed_fps = -1;
@@ -462,7 +463,8 @@ void Main::print_help(const char *p_binary) {
 	OS::get_singleton()->print("  --debug-avoidance                 Show navigation avoidance debug visuals when running the scene.\n");
 	OS::get_singleton()->print("  --debug-stringnames               Print all StringName allocations to stdout when the engine quits.\n");
 #endif
-	OS::get_singleton()->print("  --frame-delay <ms>                Simulate high CPU load (delay each frame by <ms> milliseconds).\n");
+	OS::get_singleton()->print("  --max-fps <fps>                   Set a maximum number of frames per second rendered (can be used to limit power usage). A value of 0 results in unlimited framerate.\n");
+	OS::get_singleton()->print("  --frame-delay <ms>                Simulate high CPU load (delay each frame by <ms> milliseconds). Do not use as a FPS limiter; use --max-fps instead.\n");
 	OS::get_singleton()->print("  --time-scale <scale>              Force time scale (higher values are faster, 1.0 is normal speed).\n");
 	OS::get_singleton()->print("  --disable-vsync                   Forces disabling of vertical synchronization, even if enabled in the project settings. Does not override driver-level V-Sync enforcement.\n");
 	OS::get_singleton()->print("  --disable-render-loop             Disable render loop so rendering only occurs when called explicitly from script.\n");
@@ -1325,6 +1327,17 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 				goto error;
 			}
 
+		} else if (I->get() == "--max-fps") { // set maximum rendered FPS
+
+			if (I->next()) {
+				max_fps = I->next()->get().to_int();
+				N = I->next()->next();
+			} else {
+				OS::get_singleton()->print("Missing maximum FPS argument, aborting.\n");
+				goto error;
+			}
+
+
 		} else if (I->get() == "--frame-delay") { // force frame delay
 
 			if (I->next()) {
@@ -1951,6 +1964,10 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 		OS::get_singleton()->_verbose_stdout = GLOBAL_GET("debug/settings/stdout/verbose_stdout");
 	}
 
+	if (max_fps >= 0) {
+		Engine::get_singleton()->set_max_fps(max_fps);
+	}
+
 	if (frame_delay == 0) {
 		frame_delay = GLOBAL_DEF(PropertyInfo(Variant::INT, "application/run/frame_delay_msec", PROPERTY_HINT_RANGE, "0,100,1,or_greater"), 0);
 	}

misc/dist/shell/_godot.zsh-completion

@@ -69,6 +69,7 @@ _arguments \
   '--debug-collisions[show collision shapes when running the scene]' \
   '--debug-navigation[show navigation polygons when running the scene]' \
   '--debug-stringnames[print all StringName allocations to stdout when the engine quits]' \
+  '--frame-delay[set a maximum number of frames per second rendered (can be used to limit power usage), a value of 0 results in unlimited framerate]:maximum frames per seocnd' \
   '--frame-delay[simulate high CPU load (delay each frame by the given number of milliseconds)]:number of milliseconds' \
   '--time-scale[force time scale (higher values are faster, 1.0 is normal speed)]:time scale' \
   '--disable-vsync[disable vertical synchronization even if enabled in the project settings]' \

misc/dist/shell/godot.bash-completion

@@ -72,6 +72,7 @@ _complete_godot_options() {
 --debug-collisions
 --debug-navigation
 --debug-stringnames
+--max-fps
 --frame-delay
 --time-scale
 --disable-vsync

misc/dist/shell/godot.fish

@@ -89,6 +89,7 @@ complete -c godot -l remote-debug -d "Enable remote debugging"
 complete -c godot -l debug-collisions -d "Show collision shapes when running the scene"
 complete -c godot -l debug-navigation -d "Show navigation polygons when running the scene"
 complete -c godot -l debug-stringnames -d "Print all StringName allocations to stdout when the engine quits"
+complete -c godot -l max-fps -d "Set a maximum number of frames per second rendered (can be used to limit power usage), a value of 0 results in unlimited framerate" -x
 complete -c godot -l frame-delay -d "Simulate high CPU load (delay each frame by the given number of milliseconds)" -x
 complete -c godot -l time-scale -d "Force time scale (higher values are faster, 1.0 is normal speed)" -x
 complete -c godot -l disable-render-loop -d "Disable render loop so rendering only occurs when called explicitly from script"