Always save PNG as a true colour image instead of possibly a palettised image when...

Always save PNG as a true colour image instead of possibly a palettised image when wxUSE_PALETTE is set to 0.

With wxUSE_PALETTE set to 0 and the user forcing to want a wxPNG_TYPE_PALETTE format the image would (partially) be marked as PNG_COLOR_TYPE_GRAY or PNG_COLOR_TYPE_GRAY_ALPHA instead and also saving would fail later on. Instead detect this specific case and save in the wxPNG_TYPE_COLOUR format.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66590 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775