Handle colours with alpha channel correctly in wxSVGFileDC.

Handle colours with alpha channel correctly in wxSVGFileDC.

wxColour::GetAsString(wxC2S_HTML_SYNTAX) doesn't accept colours with alpha
channel as alpha is not representable in HTML syntax, so avoid calling this
function with such colours, remove the alpha component in the caller before
using it instead.

Also slightly simplify wxBrushString() and wxPenString() functions code in
wxSVGFileDC implementation.

Closes #13214.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67883 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775