Improve method for calculation of rectangle centers

More numerically stable in the case of massive rectangles.

This is the ultimate cause behind qgis#36898 -- during rendering of the
worldwide layer in Winkel Tripel the inverse transform fails, so
the renderer sets a "maximal" filter extent for the layer (+/- double
max). The previous method for calculating the rectangles center would
fail, because it would overflow. The new method handles this case
and correctly returns 0,0.

Fixes bounding boxes show incorrectly when simplication is enabled
for a layer in certain projections.

Fixes qgis#36898
Refs qgis#30686 (partial fix -- this report includes two different bugs)

src/core/geometry/qgsrectangle.h

@@ -227,7 +227,7 @@ class CORE_EXPORT QgsRectangle
     /**
      * Returns the center point of the rectangle.
      */
-    QgsPointXY center() const { return QgsPointXY( mXmin + width() / 2, mYmin + height() / 2 ); }
+    QgsPointXY center() const { return QgsPointXY( mXmax * 0.5 + mXmin * 0.5, mYmin * 0.5 + mYmax * 0.5 ); }
 
     /**
      * Scale the rectangle around its center point.

tests/src/core/testqgsrectangle.cpp

@@ -43,6 +43,7 @@ class TestQgsRectangle: public QObject
     void scale();
     void snappedToGrid();
     void distanceToPoint();
+    void center();
 };
 
 void TestQgsRectangle::isEmpty()
@@ -404,5 +405,22 @@ void TestQgsRectangle::distanceToPoint()
   QGSCOMPARENEAR( rect.distance( QgsPointXY( 25, 115 ) ), 7.071068, 0.00001 );
 }
 
+void TestQgsRectangle::center()
+{
+  QgsRectangle rect( 10, 100, 20, 110 );
+  QCOMPARE( rect.center().x(), 15.0 );
+  QCOMPARE( rect.center().y(), 105.0 );
+  rect = QgsRectangle( 10, 100, 10, 100 );
+  QCOMPARE( rect.center().x(), 10.0 );
+  QCOMPARE( rect.center().y(), 100.0 );
+  rect = QgsRectangle( -10, -100, 10, 100 );
+  QCOMPARE( rect.center().x(), 0.0 );
+  QCOMPARE( rect.center().y(), 0.0 );
+  // a "maximal" rect
+  rect = QgsRectangle( std::numeric_limits<double>::lowest(), std::numeric_limits<double>::lowest(), std::numeric_limits<double>::max(), std::numeric_limits<double>::max() );
+  QCOMPARE( rect.center().x(), 0.0 );
+  QCOMPARE( rect.center().y(), 0.0 );
+}
+
 QGSTEST_MAIN( TestQgsRectangle )
 #include "testqgsrectangle.moc"