avoid truncating the colour channels values, this results in warnings when using VC8 debug CRT (patch 1717288) [backport from HEAD]

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@45981 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/colour.h

@@ -84,9 +84,9 @@ public:
     {
         // we don't need to know sizeof(long) here because we assume that the three
         // least significant bytes contain the R, G and B values
-        Set((ChannelType)colRGB,
-            (ChannelType)(colRGB >> 8),
-            (ChannelType)(colRGB >> 16));
+        Set((ChannelType)(0xFF & colRGB),
+            (ChannelType)(0xFF & (colRGB >> 8)),
+            (ChannelType)(0xFF & (colRGB >> 16)));
     }