BitmapColor::GetLuminance doesn't need to use unsigned long

Change-Id: Ic0f4c2c7576023a96266116a1237190586e4e816
Reviewed-on: https://gerrit.libreoffice.org/44316Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

include/vcl/salbtype.hxx

@@ -470,7 +470,9 @@ inline BitmapColor& BitmapColor::Invert()
 inline sal_uInt8 BitmapColor::GetLuminance() const
 {
     assert( !mbIndex && "Pixel represents index into colortable" );
-    return (static_cast<unsigned long>(mcBlueOrIndex) * 28UL + static_cast<unsigned long>(mcGreen) * 151UL + static_cast<unsigned long>(mcRed) * 77UL) >> 8;
+    return (static_cast<sal_uInt32>(mcBlueOrIndex) * 28
+            + static_cast<sal_uInt32>(mcGreen) * 151
+            + static_cast<sal_uInt32>(mcRed) * 77) >> 8;
 }