device: add fixed timestep policy

src/device/device.cpp

@@ -370,6 +370,9 @@ bool Device::frame()
 		RECORD_FLOAT("device.smoothed_dt", smoothed_dt);
 		RECORD_FLOAT("device.smoothed_fps", 1.0f/smoothed_dt);
 		dt = smoothed_dt;
+	} else if (_timestep_policy == TimestepPolicy::FIXED) {
+		// TODO
+		dt = raw_dt;
 	} else {
 		dt = raw_dt;
 	}

src/device/types.h

@@ -87,6 +87,7 @@ struct TimestepPolicy
 	{
 		VARIABLE,
 		SMOOTHED,
+		FIXED,
 
 		COUNT
 	};

src/lua/lua_api.cpp

@@ -154,6 +154,7 @@ static const TimestepPolicyInfo s_timestep_policy[] =
 {
 	{ STRING_ID_32("variable", UINT32_C(0xd638145a)), TimestepPolicy::VARIABLE },
 	{ STRING_ID_32("smoothed", UINT32_C(0x460246ef)), TimestepPolicy::SMOOTHED },
+	{ STRING_ID_32("fixed", UINT32_C(0x08eeb19e)),    TimestepPolicy::FIXED    }
 };
 CE_STATIC_ASSERT(countof(s_timestep_policy) == TimestepPolicy::COUNT);