Compare scaled images approximately in the unit tests.

Comparing them exactly results in tests failures when using compiler version
different from the one that was used to generate the test fails, see
http://thread.gmane.org/gmane.comp.lib.wxwidgets.devel/151149/focus=151154
and other messages in the same thread.

tests/image/image.cpp

@@ -1340,6 +1340,39 @@ void ImageTestCase::BMPFlippingAndRLECompression()
 }
 
 
+static bool
+CompareApprox(const wxImage& i1, const wxImage& i2)
+{
+    if ( i1.GetWidth() != i2.GetWidth() )
+        return false;
+
+    if ( i1.GetHeight() != i2.GetHeight() )
+        return false;
+
+    const unsigned char* p1 = i1.GetData();
+    const unsigned char* p2 = i2.GetData();
+    const int numBytes = i1.GetWidth()*i1.GetHeight()*3;
+    for ( int n = 0; n < numBytes; n++, p1++, p2++ )
+    {
+        switch ( *p1 - *p2 )
+        {
+            case -1:
+            case  0:
+            case +1:
+                // Accept up to one pixel difference, this happens because of
+                // different rounding behaviours in different compiler versions
+                // even under the same architecture, see the example in
+                // http://thread.gmane.org/gmane.comp.lib.wxwidgets.devel/151149/focus=151154
+                break;
+
+            default:
+                return false;
+        }
+    }
+
+    return true;
+}
+
 // The 0 below can be replaced with 1 to generate, instead of comparing with,
 // the test files.
 #define ASSERT_IMAGE_EQUAL_TO_FILE(image, file) \
@@ -1351,7 +1384,11 @@ void ImageTestCase::BMPFlippingAndRLECompression()
     { \
         wxImage imageFromFile(file); \
         CPPUNIT_ASSERT_MESSAGE( "Failed to load " file, imageFromFile.IsOk() ); \
-        CPPUNIT_ASSERT_EQUAL( imageFromFile, image ); \
+        CPPUNIT_ASSERT_MESSAGE \
+        ( \
+            "Wrong scaled " + CppUnit::assertion_traits<wxImage>::toString(image), \
+            CompareApprox(imageFromFile, image) \
+        ); \
     }
 
 void ImageTestCase::ScaleCompare()