plotting - Removing unwanted appearance of underlying mesh

Let us first define two positive definite matrices:

M1 = {{2, -6}, {4, 8}};
M2 = {{2, 3}, {4, 8}};

then set two points p1={-1,-1} and p2={1,1}. Finally we define an anisotropic distance function, namely:

d[q1_, q2_, M_] := Sqrt[(q1 - q2).M.(q1 - q2)]

When trying to plot the anisotropic Voronoi cells as follows:

Show[
 Graphics[Point[{p1, p2}]],
 RegionPlot[
  {
   d[{x, y}, p1, M1] < d[{x, y}, p2, M2],
   d[{x, y}, p1, M1] > d[{x, y}, p2, M2]
   },

  {x, -4, 4}, {y, -4, 4}
  ]
 ]

I obtain the following image:

My question is: How can I get rid of the underlying mesh, which is visible in this example?

Two remarks:

1. Removing the points' plotting also removes the mesh.



2. Adding something like Mesh->None to the RegionPlot doesn't help.

---

Edit:

It seems this problem is specific to Mac OS X. Here is the Options[RegionPlot] output:

{AlignmentPoint -> Center, AspectRatio -> 1, Axes -> False, 
 AxesLabel -> None, AxesOrigin -> Automatic, AxesStyle -> {}, 
 Background -> None, BaselinePosition -> Automatic, BaseStyle -> {}, 
 BoundaryStyle -> Automatic, ColorFunction -> Automatic, 
 ColorFunctionScaling -> True, ColorOutput -> Automatic, 

 ContentSelectable -> Automatic, CoordinatesToolOptions -> Automatic, 
 DisplayFunction :> $DisplayFunction, Epilog -> {}, 
 Evaluated -> Automatic, EvaluationMonitor -> None, 
 FormatType :> TraditionalForm, Frame -> True, FrameLabel -> None, 
 FrameStyle -> {}, FrameTicks -> Automatic, FrameTicksStyle -> {}, 
 GridLines -> None, GridLinesStyle -> {}, ImageMargins -> 0.`, 
 ImagePadding -> All, ImageSize -> Automatic, 
 ImageSizeRaw -> Automatic, LabelStyle -> {}, 
 MaxRecursion -> Automatic, Mesh -> None, 
 MeshFunctions -> {#1 &, #2 &}, MeshShading -> None, 

 MeshStyle -> Automatic, Method -> Automatic, 
 PerformanceGoal :> $PerformanceGoal, PlotLabel -> None, 
 PlotPoints -> Automatic, PlotRange -> Full, 
 PlotRangeClipping -> True, PlotRangePadding -> Automatic, 
 PlotRegion -> Automatic, PlotStyle -> Automatic, 
 PreserveImageOptions -> Automatic, Prolog -> {}, RotateLabel -> True,
  TextureCoordinateFunction -> Automatic, 
 TextureCoordinateScaling -> Automatic, Ticks -> Automatic, 
 TicksStyle -> {}, WorkingPrecision -> MachinePrecision}

Answer

You are combining the images in the form

Show[Graphics[simplePrimitives], complicatedRegionPlot]

The options in the resulting figure are inherited from the first term, namely Graphics[simplePrimitives]. This does not include the "TransparentPolygonMesh" -> True generated by RegionPlot. You see the mesh as a result. If you combine things as follows:

Show[complicatedRegionPlot, Graphics[simplePrimitives]]

Then the resulting image will have the standard RegionPlot options and you'll no longer see the mesh.

I think the preferred way to do this, however, is to use Epilog, as in J.M.'s response.