Avoid accessing instance variables in inline functions when compiling…

… with msvc (#177)

* Rename `DPSshfill` to `DPSshfill_`

This method takes one argument, so by convention, the name of the variable in the method table should end with a single underscore

* DPSOperators.h: Use macros to define inline functions

* Don't access instance variables when compiling with Visual Studio

Headers/Additions/GNUstepGUI/GSMethodTable.h

@@ -112,7 +112,7 @@ typedef struct {
         (NSGraphicsContext*, SEL, const char *, size_t);
   void (*GSShowGlyphs__)
         (NSGraphicsContext*, SEL, const NSGlyph *, size_t);
-  void (*GSShowGlyphsWithAdvances__)
+  void (*GSShowGlyphsWithAdvances___)
         (NSGraphicsContext*, SEL, const NSGlyph *, const NSSize *, size_t);
 
 /* ----------------------------------------------------------------------- */
@@ -313,7 +313,7 @@ typedef struct {
   struct breaks apps that use PS* or DPS* functions and were compiled with
   an earlier version, so it's here until we figure out how to handle that.
   */
-  void (*DPSshfill)
+  void (*DPSshfill_)
         (NSGraphicsContext*, SEL, NSDictionary *);
 } gsMethodTable;