Path tracer offline mode override

[jakubtomsu]

This overrides the scene view's offline mode, which changes how path tracer is invalidated. It's not needed for the simple path tracer example but can be useful for some experimental features. For example, you can use this to do manual motion blur, because in this mode the path tracer samples don't get invalidated on camera or object mvement.

[mikeroberts3000]

This PR is great. Here is my proposed design. I'm using UFUNCTIONS instead of UPROPERTIES to make it more clear that they set a piece of deferred state that gets consumed differently to the rest of the state in the capture component. I'm also slightly simplifying existing code here because I just realized that setupView() executes on the game thread, so there is no need for any synchronization primitives on any of the deferred state:

    // SpSceneCaptureComponent.h

    // ...

    // set deferred state to be consumed in an FSceneViewExtension callback

    UFUNCTION(BlueprintCallable, CallInEditor, Category="SPEAR")
    void RequestPathTracerReset();

    UFUNCTION(BlueprintCallable, Category="SPEAR")
    void RequestSetOfflineRender(bool bOverrideIsOfflineRender, bool bIsOfflineRender);

    // ...

    // deferred state to be consumed in an FSceneViewExtension callback
    bool request_path_tracer_reset_ = false;
    bool request_override_is_offline_render_ = false;
    bool request_is_offline_render_ = false;

// SpSceneCaptureComponent.cpp

void USpSceneCaptureComponent2D::Initialize()
{
    // ...

    request_path_tracer_reset_ = false;
    request_override_is_offline_render_ = false;
    request_is_offline_render_ = false;
}

void USpSceneCaptureComponent2D::RequestPathTracerReset()
{
    SP_ASSERT(IsInitialized());
    request_path_tracer_reset_ = true;
}

void USpSceneCaptureComponent2D::RequestSetPathTracerIsOfflineRender(bool bOverrideIsOfflineRender, bool bIsOfflineRender)
{
    SP_ASSERT(IsInitialized());
    request_override_is_offline_render_ = bOverrideIsOfflineRender;
    request_is_offline_render_ = bIsOfflineRender;
}

void USpSceneCaptureComponent2D::setupView(FSceneViewFamily& view_family, FSceneView& view)
{
    const TArray<FEngineShowFlagsSetting>& engine_show_flag_settings = GetShowFlagSettings();

    for (auto& engine_show_flag_setting : engine_show_flag_settings) {
        if (Unreal::toStdString(engine_show_flag_setting.ShowFlagName) == "LightingOnlyOverride" && engine_show_flag_setting.Enabled) {
            view.DiffuseOverrideParameter = FVector4f(GEngine->LightingOnlyBrightness.R, GEngine->LightingOnlyBrightness.G, GEngine->LightingOnlyBrightness.B, 0.0f);
            view.SpecularOverrideParameter = FVector4f(0.0f, 0.0f, 0.0f, 0.0f);
        }

        if (Unreal::toStdString(engine_show_flag_setting.ShowFlagName) == "Specular" && !engine_show_flag_setting.Enabled) {
            view.SpecularOverrideParameter = FVector4f(0.0f, 0.0f, 0.0f, 0.0f);
        }
    }

    if (request_path_tracer_reset_) {
        view.bForcePathTracerReset = true;
        request_path_tracer_reset_ = false;
    }

    if (request_override_is_offline_render_) {
        view.bIsOfflineRender = request_is_offline_render_;
    }
}

[jakubtomsu]

Okay that makes sense

[mikeroberts3000]

Please implement my proposed changes before merging.