Optimize selecting font in generated PostScript code

Emit code to select PostScript font only if it is needed (not at every call to SetFont(), but only if font has been really changed prior to a text drawing operation).

interface/wx/dcps.h

@@ -12,6 +12,13 @@
     can write PostScript files on any platform. See wxDC for descriptions of
     the member functions.
 
+    @section start_doc Starting a document
+
+    Document should be started with call to StartDoc() prior to calling any
+    function to execute a drawing operation.
+    However, some functions, like SetFont(), may be legitimately called even
+    before StartDoc().
+
     @library{wxbase}
     @category{dc}
 */