ofz#968 short-cut slow create mask when we are our own mask already

Change-Id: I5969226bf2da34d34e30e44ae763792cdc072bc8
Reviewed-on: https://gerrit.libreoffice.org/35921Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/gdi/bitmap.cxx

@@ -1107,9 +1107,16 @@ bool Bitmap::Expand( sal_uLong nDX, sal_uLong nDY, const Color* pInitColor )
 
 Bitmap Bitmap::CreateMask( const Color& rTransColor, sal_uLong nTol ) const
 {
-    Bitmap              aNewBmp( GetSizePixel(), 1 );
-    ScopedReadAccess    pReadAcc(const_cast<Bitmap&>(*this));
+    ScopedReadAccess pReadAcc(const_cast<Bitmap&>(*this));
+
+    if (!nTol && (pReadAcc->GetScanlineFormat() == ScanlineFormat::N1BitLsbPal || pReadAcc->GetScanlineFormat() == ScanlineFormat::N1BitMsbPal)
+        && pReadAcc->GetBestMatchingColor(Color(COL_WHITE)) == pReadAcc->GetBestMatchingColor(rTransColor))
+    {
+        //if we're a 1 bit pixel already, and the transcolor matches the color that would replace it already already, then just return a copy
+        return *this;
+    }
 
+    Bitmap              aNewBmp(GetSizePixel(), 1);
     ScopedWriteAccess   pWriteAcc(aNewBmp);
     bool                bRet = false;