NSGraphicsContext: Add methods method (#179)

gnustep-back uses the `NSGraphicsContext->methods` _variable_, but using an instance variable across module boundaries is not supported when building with Visual Studio.

This commits adds a `[NSGraphicsContext methods]` _method_, which can be accessed across module boundaries (and hence when building with Visual Studio).

Headers/AppKit/NSGraphicsContext.h

@@ -211,6 +211,7 @@ APPKIT_EXPORT_CLASS
                                              flipped: (BOOL)flipped;
 #endif
 
+- (const gsMethodTable *) methods;
 - (NSDictionary *) attributes;
 - (void *) graphicsPort;
 

Source/NSGraphicsContext.m

@@ -375,6 +375,11 @@ NSGraphicsContext	*GSCurrentContext(void)
   return context_info;
 }
 
+- (const gsMethodTable *) methods
+{
+  return methods;
+}
+
 - (void) flushGraphics
 {
   [self subclassResponsibility: _cmd];