Add wx{Pen,Brush}::Is[Non]Transparent() methods and use them.

Add wx{Pen,Brush}::Is[Non]Transparent() methods and use them.

Using GetStyle() == wx{PEN,BRUSH}STYLE_TRANSPARENT doesn't work for
uninitialized pen or brush objects so add convenient helpers which do work for
them.

Use the new helper functions everywhere instead of explicitly checking for
style. This makes the code shorter and more clear and also fixes some bugs (at
least those in GTK printing code).

Notice that this patch removes the main reason for explicitly initializing
m_pen and m_brush in wxGTKDCImpl ctor but this initialization still can't be
removed, at least for the latter, as doing this somehow breaks GetPixel(). It
would be nice to understand why and do remove this initialization so that a
newly created DC doesn't have any non-default pen nor brush.

Closes #12522.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65820 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775