do not shift viewport pixels on OpenGL

src/drawing/IDrawingEngine.h

@@ -23,6 +23,16 @@
 struct rct_drawpixelinfo;
 interface IDrawingContext;
 
+enum DRAWING_ENGINE_FLAGS
+{
+    DEF_NONE = 0,
+
+    /**
+     * Whether or not the engine will only draw changed blocks of the screen each frame.
+     */
+    DEF_DIRTY_OPTIMISATIONS = 1 << 0,
+};
+
 interface IDrawingEngine
 {
     virtual ~IDrawingEngine() { }
@@ -36,6 +46,8 @@ interface IDrawingEngine
     virtual sint32  Screenshot() abstract;
 
     virtual IDrawingContext * GetDrawingContext(rct_drawpixelinfo * dpi) abstract;
+
+    virtual DRAWING_ENGINE_FLAGS GetFlags() abstract;
 };
 
 namespace DrawingEngineFactory

src/drawing/NewDrawing.cpp

@@ -66,6 +66,16 @@ extern "C"
         _drawingEngine = nullptr;
     }
 
+    bool drawing_engine_has_dirty_optimisations()
+    {
+        bool result = false;
+        if (_drawingEngine != nullptr)
+        {
+            result = (_drawingEngine->GetFlags() & DEF_DIRTY_OPTIMISATIONS);
+        }
+        return result;
+    }
+
     void gfx_set_dirty_blocks(sint16 left, sint16 top, sint16 right, sint16 bottom)
     {
         if (_drawingEngine != nullptr)

src/drawing/NewDrawing.h

@@ -27,6 +27,8 @@ void drawing_engine_set_palette(SDL_Color * colours);
 void drawing_engine_draw();
 void drawing_engine_dispose();
 
+bool drawing_engine_has_dirty_optimisations();
+
 #ifdef _cplusplus
 }
 #endif

src/drawing/engines/OpenGLDrawingEngine.cpp

@@ -202,6 +202,11 @@ public:
         return _drawingContext;
     }
 
+    DRAWING_ENGINE_FLAGS GetFlags() override
+    {
+        return DEF_NONE;
+    }
+
     rct_drawpixelinfo * GetDPI()
     {
         return &_bitsDPI;

src/drawing/engines/SoftwareDrawingEngine.cpp

@@ -327,6 +327,11 @@ public:
         return _drawingContext;
     }
 
+    DRAWING_ENGINE_FLAGS GetFlags() override
+    {
+        return DEF_DIRTY_OPTIMISATIONS;
+    }
+
     rct_drawpixelinfo * GetDPI()
     {
         return &_bitsDPI;