Rename FPS limiting project setting and Engine property to be easier to discover

[Calinou]

Related to #2001.

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

As documented in godotengine/godot-docs#6186, recent research in reducing input lag in games has found two ways to reduce input latency while also reducing GPU power draw:

• When using a variable refresh rate monitor (G-Sync/FreeSync), V-Sync should be enabled and the framerate should be limited to a slightly lower value than the monitor's maximum refresh rate.
  • To lower latency, this is best accomplished with a built-in FPS limiter, rather than an external tool. To this end, Godot should make it easy for project developers to configure this value (and then expose a FPS limit setting for players to configure).
• When not using a variable refresh rate monitor, V-Sync can be disabled and the framerate can be limited to a value slightly greater than the monitor's refresh rate. This will lead to tearing, but will have lower latency than with V-Sync enabled, which can be preferable in some games.
  • Compared to running with no V-Sync and uncapped FPS, this can lower input lag significantly in GPU-bound scenarios by avoiding pipeline stalls while also making the average input lag more consistent (which is important in competitive scenarios). If the FPS limit is well-chosen by the player, sudden framerate jumps will no longer occur depending on scene complexity. Those framerate jumps lead to unexpected variance in input lag.
  • To lower latency, this must be accomplished with a built-in FPS limiter. External FPS limiters will not have any positive impact on latency in this scenario. See Less invsive option that works in all games? ishitatsuyuki/LatencyFleX#4.

However, limiting framerate to a set value in Godot is not as obvious as in other engines, due to the project setting and engine property having unintuitive names:

• The Debug > Settings > FPS > Force FPS project setting was moved to the Debug section in 3.0 by godotengine/godot@920310e72a.
  • The Application > Run > Frame Delay Msec is mentioned as an alternative, but it is not designed to provide accurate FPS limiting. Instead, frame delay is only meant to simulate CPU-constrained scenarios (therefore leading to stuttering with values that are close to the monitor's refresh rate in milliseconds-per-frame).
• The Engine singleton has a target_fps project setting, but this is actually a maximum framerate, not a target for dynamic resolution or automatic graphics settings adjustment.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Rename FPS limiting project setting and Engine property to be easier to discover.

Note: This proposal does not propose changing any default settings' values, only their names.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

• Rename Debug > Settings > FPS > Force FPS project setting to Application > Run > Max FPS to put it in the same section as Frame Delay Msec.
• Rename Engine's target_fps to max_fps, which is consistent with most other engines (Unreal, Source, id Tech, …). Unity does seem to be using targetFrameRate still, but I think this naming would be best left unused to prepare for a future implementation of dynamic resolution scaling.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No.

Is there a reason why this should be core and not an add-on in the asset library?

Developers should have access to ways to minimize input lag in their projects as easily as possible.