A depth peeling fix when compositing

When compositing we need the alpha to be correct when it
comes out of depth peeling. Previously it was being set
to 1.0.

Rendering/OpenGL2/glsl/vtkDepthPeelingPassFinalFS.glsl

@@ -29,6 +29,6 @@ void main()
 {
   vec4 t1Color = texture2D(translucentRGBATexture, tcoordVC);
   vec4 t2Color = texture2D(opaqueRGBATexture, tcoordVC);
-  gl_FragData[0].a = 1.0;
+  gl_FragData[0].a = t1Color.a +  (1.0-t1Color.a)*t2Color.a;
   gl_FragData[0].rgb = (t1Color.rgb*t1Color.a + t2Color.rgb*(1.0-t1Color.a));
 }

Rendering/OpenGL2/vtkDepthPeelingPass.cxx

@@ -453,6 +453,7 @@ void vtkDepthPeelingPass::Render(const vtkRenderState *s)
   bool done = false;
   GLuint nbPixels = threshold + 1;
   int peelCount = 0;
+  glDepthFunc( GL_LEQUAL );
   while(!done)
     {
     glDepthMask(GL_TRUE);