Add heuristic to not simplify geometries for nothing

Tested with a release binary and Swiss OSM roads in PostGIS at several zoom
levels (the higher, the less features), values in ms (average over at
least 100 queries):

Level | Before | After | % improvement
-------------------------------------------
7     |  37    |  36   | 2.7
6     |  41    |  38   | 7.32
5     |  48    |  46   | 4.17
4     |  70    |  64   | 8.57
3     | 114    | 109   | 4.39

Not much improvement (I'm quite dispointed, here), but not much change in
the code.

src/core/qgsmaptopixelgeometrysimplifier.cpp

@@ -547,9 +547,15 @@ bool QgsMapToPixelSimplifier::simplifyPoints( QgsWkbTypes::Type wkbType, QgsCons
   if ( numPoints <= ( isaLinearRing ? 6 : 3 ) )
     return false;
 
-  QgsRectangle envelope = calculateBoundingBox( singleType, QgsConstWkbPtr( sourceWkbPtr ), numPoints );
-  sourceWkbPtr -= sizeof( int );
+  const QgsRectangle envelope = calculateBoundingBox( singleType, QgsConstWkbPtr( sourceWkbPtr ), numPoints );
 
+  if ( qMax( envelope.width(), envelope.height() ) / numPoints > tolerance * 2.0 )
+  {
+    //points are in average too far appart to lead to any significant simplification
+    return false;
+  }
+
+  sourceWkbPtr -= sizeof( int );
   int targetWkbSize = 5 + sizeof( int ) + numPoints * ( 2 * sizeof( double ) );
   unsigned char* targetWkb = new unsigned char[ targetWkbSize ];