Fix: [SDL2] simplify what to redraw to prevent tearing (#8685)

When there are a lot of rects to redraw, of which one of the last
ones is almost the full screen, visual tearing happens over the
vertical axis. This is most visible when scrolling the map.

This can be prevented by using less rects. To simplify the situation,
and as solutions like OpenGL need this anyway, keep a single rect
that shows the biggest size that updates everything correctly.

Although this means it needs a bit more time redrawing where it
is strictly seen not needed, it also means less commands have
to be executed in the backend. In the end, this is a trade-off,
and from experiments it seems the approach of this commit gives
a better result.

src/core/geometry_func.cpp

@@ -26,3 +26,25 @@ Dimension maxdim(const Dimension &d1, const Dimension &d2)
 	d.height = std::max(d1.height, d2.height);
 	return d;
 }
+
+/**
+ * Compute the bounding rectangle around two rectangles.
+ * @param r1 First rectangle.
+ * @param r2 Second rectangle.
+ * @return The bounding rectangle, the smallest rectangle that contains both arguments.
+ */
+Rect BoundingRect(const Rect &r1, const Rect &r2)
+{
+	/* If either the first or the second is empty, return the other. */
+	if (IsEmptyRect(r1)) return r2;
+	if (IsEmptyRect(r2)) return r1;
+
+	Rect r;
+
+	r.top    = std::min(r1.top,    r2.top);
+	r.bottom = std::max(r1.bottom, r2.bottom);
+	r.left   = std::min(r1.left,   r2.left);
+	r.right  = std::max(r1.right,  r2.right);
+
+	return r;
+}