fix transparency handling in BitmapEx::GetPixelColor()

Apparently the Color class uses transparency as the opposite
of the normally used meaning of opacity, so transparency 255
means transparent.

Change-Id: I3a76c2c93d98ad3c850017d3fd569b04dc6c19c8
Reviewed-on: https://gerrit.libreoffice.org/69211
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

sw/qa/extras/ooxmlexport/ooxmlexport10.cxx

@@ -585,7 +585,7 @@ DECLARE_OOXMLEXPORT_TEST(testMsoBrightnessContrast, "msobrightnesscontrast.docx"
     CPPUNIT_ASSERT_EQUAL(58L, aBitmap.GetSizePixel().Width());
     CPPUNIT_ASSERT_EQUAL(320L, aBitmap.GetSizePixel().Height());
     Color aColor(aBitmap.GetPixelColor(20, 30));
-    CPPUNIT_ASSERT_EQUAL(Color( 255, 0xce, 0xce, 0xce ), aColor);
+    CPPUNIT_ASSERT_EQUAL(Color( 0xce, 0xce, 0xce ), aColor);
 }
 
 DECLARE_OOXMLEXPORT_TEST(testChartSize, "chart-size.docx")

vcl/source/gdi/bitmapex.cxx

@@ -758,7 +758,7 @@ Color BitmapEx::GetPixelColor(sal_Int32 nX, sal_Int32 nY) const
         aColor.SetTransparency( pAlphaReadAccess->GetPixel( nY, nX ).GetIndex() );
     }
     else
-        aColor.SetTransparency(255);
+        aColor.SetTransparency( 0 );
     return aColor;
 }