create "disabled" image (icon) only if the original changes

Currently we create a "disabled" version of  bitmaps in "Image"
class (used for toolbar icons for example) in every draw call.
This is not really optimal. The solution is to calculate the
checksum of the original and create a "disabled" version only if
the checksum changes.

Change-Id: I6ed2d08c64fee028b479fedb35e4384f112e98e2

vcl/inc/image.h

@@ -39,6 +39,8 @@ public:
 private:
 
     BitmapEx    maBmpEx;
+    BitmapChecksum maBitmapChecksum;
+
     BitmapEx    maDisabledBmpEx;
     BitmapEx*   mpDisplayBmp;
     Size        maSize;

vcl/source/gdi/impimage.cxx

@@ -182,7 +182,12 @@ void ImplImageBmp::Draw( OutputDevice* pOutDev,
 
         if( nStyle & DrawImageFlags::Disable )
         {
-            ImplUpdateDisabledBmpEx();
+            BitmapChecksum aChecksum = maBmpEx.GetChecksum();
+            if (maBitmapChecksum != aChecksum)
+            {
+                maBitmapChecksum = aChecksum;
+                ImplUpdateDisabledBmpEx();
+            }
             pOutDev->DrawBitmapEx( rPos, aOutSize, aSrcPos, maSize, maDisabledBmpEx );
         }
         else