- Modified commit from @OrdinaryMagician #1261

- Always allow skyboxes by default
- Add option to disable skyboxes through mapinfo
Squashed commit of the following:

commit d4c4d9310d0157ce575f52989b5b080398cf7629
Author: Marisa Kirisame <marisa@sayachan.org>
Date:   Wed Dec 30 09:58:39 2020 +0100

    Disable AO for skybox portals (can be forced back with MAPINFO flag).

src/rendering/hwrenderer/scene/hw_drawinfo.cpp

@@ -672,7 +672,7 @@ void HWDrawInfo::DrawScene(int drawmode)
 	}
 	else if (drawmode == DM_PORTAL && ssao_portals_available > 0)
 	{
-		applySSAO = true;
+		applySSAO = (mCurrentPortal->AllowSSAO() || Level->flags3&LEVEL3_SKYBOXAO);
 		ssao_portals_available--;
 	}
 

src/rendering/hwrenderer/scene/hw_portal.cpp

@@ -718,6 +718,7 @@ void HWSkyboxPortal::Shutdown(HWDrawInfo *di, FRenderState &rstate)
 }
 
 const char *HWSkyboxPortal::GetName() { return "Skybox"; }
+bool HWSkyboxPortal::AllowSSAO() { return false; }	// [MK] sector skyboxes don't allow SSAO by default
 
 //-----------------------------------------------------------------------------
 //-----------------------------------------------------------------------------

src/rendering/hwrenderer/scene/hw_portal.h

@@ -77,6 +77,7 @@ public:
     virtual line_t *ClipLine() { return nullptr; }
 	virtual void * GetSource() const = 0;	// GetSource MUST be implemented!
 	virtual const char *GetName() = 0;
+	virtual bool AllowSSAO() { return true; }
 	virtual bool IsSky() { return false; }
 	virtual bool NeedCap() { return true; }
 	virtual bool NeedDepthBuffer() { return true; }
@@ -258,6 +259,7 @@ protected:
 	virtual void * GetSource() const { return portal; }
 	virtual bool IsSky() { return true; }
 	virtual const char *GetName();
+	virtual bool AllowSSAO() override;
 
 public: