Change how dirty screen, window and viewport areas are tracked for later redrawing

Track dirty viewport areas seperately form general screen redraws.
Maintain a dirty block grid per viewport, with a smaller block size.
Use even smaller block size in viewport map mode.

Use a rectangle array for general screen redraws instead of a block grid.

Add a dirty bit to windows and widgets, to simplify the common case
of repainting a whole window or widget, without catching
neighbouring windows or viewports.

src/gfx_func.h

@@ -21,7 +21,7 @@
  * (this is btw. also possible if needed). This is used to avoid a
  * flickering of the screen by the video driver constantly repainting it.
  *
- * This whole mechanism is controlled by an rectangle defined in #_invalid_rect. This
+ * This whole mechanism was controlled by an rectangle defined in #_invalid_rect. This
  * rectangle defines the area on the screen which must be repaint. If a new object
  * needs to be repainted this rectangle is extended to 'catch' the object on the
  * screen. At some point (which is normally uninteresting for patch writers) this
@@ -32,7 +32,6 @@
  * rectangle information. Then a new round begins by marking objects "dirty".
  *
  * @see VideoDriver::MakeDirty
- * @see _invalid_rect
  * @see _screen
  */
 
@@ -134,6 +133,7 @@ const char *GetCharAtPosition(const char *str, int x, FontSize start_fontsize =
 
 void DrawDirtyBlocks();
 void SetDirtyBlocks(int left, int top, int right, int bottom);
+void UnsetDirtyBlocks(int left, int top, int right, int bottom);
 void MarkWholeScreenDirty();
 
 void GfxInitPalettes();