0023272: Image comparison algorithm

A new class Image_Diff for comparison of images
and a draw-command "diffimage", which compares 2 images.

Image_PixMap redesigned to provide interface for low-level image operations.
New Image_AlienPixMap class now intended for Save/Load functionality.

Aspect_PixMap class dropped.
Xw_PixMap and WNT_PixMap classes now do not inherit from Aspect_PixMap and deprecated.

ToPixMap methods now retrieve Image_PixMap as argument.

Conflicts:
src/ViewerTest/ViewerTest.cxx
Remarks applied
Fix compilation (correct merging error)

Eliminated Aspect <-> Image cyclic dependency

Fixed GIF dump in case of BGR32 image format

src/Image/FILES

@@ -6,5 +6,11 @@ Image_PixelAddress.cxx
 Image_PixelAddress.hxx
 Image.edl
 Image_CMPLRS.edl
-Image_CRawBufferData.hxx
-Image_HPrivateImage.hxx
+Image_PixMap.hxx
+Image_PixMap.cxx
+Image_PixMapData.hxx
+Image_Color.hxx
+Image_AlienPixMap.hxx
+Image_AlienPixMap.cxx
+Image_Diff.hxx
+Image_Diff.cxx