Simplify SDL gr_flip: nullptr implies copy all

Per the header comments, passing nullptr for source or destination
copies the entire region.  Use that instead of explicitly requesting the
entire region.  This should be more efficient, since it bypasses the
source rectangle clipping code that SDL would otherwise run.

similar/arch/sdl/gr.cpp

@@ -43,13 +43,7 @@ static int gr_installed;
 
 void gr_flip()
 {
-	SDL_Rect src, dest;
-
-	dest.x = src.x = dest.y = src.y = 0;
-	dest.w = src.w = canvas->w;
-	dest.h = src.h = canvas->h;
-
-	SDL_BlitSurface(canvas, &src, screen, &dest);
+	SDL_BlitSurface(canvas, nullptr, screen, nullptr);
 	SDL_Flip(screen);
 }