Fix clean build: add RCreateContextWayland to wraster.h.in

wraster.h is generated from wraster.h.in — the declaration was only
in the generated file, not the template. Also includes the Wayland
no-op for RConvertImage/RConvertImageMask (context->dpy == NULL).

wrlib/wraster.h.in

@@ -356,6 +356,9 @@ RContext *RCreateContext(Display *dpy, int screen_number,
                          const RContextAttributes *attribs)
 	__wrlib_useresult __wrlib_nonalias __wrlib_nonnull(1);
 
+RContext *RCreateContextWayland(int width, int height,
+                                const RContextAttributes *attribs);
+
 void RDestroyContext(RContext *context);
 
 Bool RGetClosestXColor(RContext *context, const RColor *color, XColor *retColor)