move cairo extract bitmap code from canvas to vcl

part of making GetMask/GetAlpha an internal detail of vcl

Change-Id: I45c2e9fdae08d7f444a64e8e04a6f65bb525cbd1
Reviewed-on: https://gerrit.libreoffice.org/51417Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

include/vcl/BitmapTools.hxx

@@ -98,6 +98,12 @@ VCL_DLLPUBLIC void DrawAndClipBitmap(const Point& rPos, const Size& rSize, const
 
 VCL_DLLPUBLIC css::uno::Sequence< sal_Int8 > GetMaskDIB(BitmapEx const & aBmpEx);
 
+/**
+ * @param data will be filled with alpha data, if xBitmap is alpha/transparent image
+ * @param bHasAlpha will be set to true if resulting surface has alpha
+ **/
+VCL_DLLPUBLIC void CanvasCairoExtractBitmapData( BitmapEx & rBmpEx, Bitmap & rBitmap, unsigned char*& data, bool& bHasAlpha );
+
 }} // end vcl::bitmap
 
 #endif // INCLUDED_VCL_BITMAP_TOOLS_HXX