Add wxIMAGE_OPTION_GIF_TRANSPARENCY for GIF image loading

Allow to keep the originally defined transparent pixels colour instead
of replacing it with bright pink (which is still the default behaviour).

Closes #18014.

include/wx/imaggif.h

@@ -20,6 +20,10 @@
 
 #define wxIMAGE_OPTION_GIF_COMMENT wxT("GifComment")
 
+#define wxIMAGE_OPTION_GIF_TRANSPARENCY           wxS("Transparency")
+#define wxIMAGE_OPTION_GIF_TRANSPARENCY_HIGHLIGHT wxS("Highlight")
+#define wxIMAGE_OPTION_GIF_TRANSPARENCY_UNCHANGED wxS("Unchanged")
+
 struct wxRGB;
 struct GifHashTableType;
 class WXDLLIMPEXP_FWD_CORE wxImageArray; // anidecod.h