Make wxBitmap::ConvertToDisabled() available in all ports.

This method was defined in wxBitmapBase which is not used by wxMSW (and wxOS2)
so it wasn't available there. Move the definition of the method inline and
reuse it for all ports, making it part of either wxBitmapBase or wxBitmap as
appropriate.

This is clearly ugly but we still have no good solution for deriving wxBitmap
from wxBitmapBase in wxMSW as it already inherits from MSW-specific wxGDIImage
there.

Also document that ConvertToDisabled() is only available when wxUSE_IMAGE==1.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66086 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/bitmap.h

@@ -19,11 +19,11 @@
 #include "wx/string.h"
 #include "wx/gdicmn.h"  // for wxBitmapType
 #include "wx/colour.h"
+#include "wx/image.h"
 
 class WXDLLIMPEXP_FWD_CORE wxBitmap;
 class WXDLLIMPEXP_FWD_CORE wxBitmapHandler;
 class WXDLLIMPEXP_FWD_CORE wxIcon;
-class WXDLLIMPEXP_FWD_CORE wxImage;
 class WXDLLIMPEXP_FWD_CORE wxMask;
 class WXDLLIMPEXP_FWD_CORE wxPalette;
 
@@ -168,6 +168,9 @@ public:
 
 #if wxUSE_IMAGE
     virtual wxImage ConvertToImage() const = 0;
+
+    // Convert to disabled (dimmed) bitmap.
+    wxBitmap ConvertToDisabled(unsigned char brightness = 255) const;
 #endif // wxUSE_IMAGE
 
     virtual wxMask *GetMask() const = 0;
@@ -175,9 +178,6 @@ public:
 
     virtual wxBitmap GetSubBitmap(const wxRect& rect) const = 0;
 
-    // Convert to disabled (dimmed) bitmap.
-    wxBitmap ConvertToDisabled(unsigned char brightness = 255) const;
-
     virtual bool SaveFile(const wxString &name, wxBitmapType type,
                           const wxPalette *palette = NULL) const = 0;
     virtual bool LoadFile(const wxString &name, wxBitmapType type) = 0;
@@ -272,6 +272,20 @@ protected:
     #include "wx/os2/bitmap.h"
 #endif
 
+#if wxUSE_IMAGE
+inline
+wxBitmap
+#if wxUSE_BITMAP_BASE
+wxBitmapBase::
+#else
+wxBitmap::
+#endif
+ConvertToDisabled(unsigned char brightness) const
+{
+    return ConvertToImage().ConvertToDisabled(brightness);
+}
+#endif // wxUSE_IMAGE
+
 // we must include generic mask.h after wxBitmap definition
 #if defined(__WXMGL__) || defined(__WXDFB__)
     #define wxUSE_GENERIC_MASK 1