Add and use SDL_FALLTHROUGH for fallthroughs

Case fallthrough warnings can be suppressed using the __fallthrough__
compiler attribute. Unfortunately, not all compilers have this
attribute, or even have __has_attribute to check if they have the
__fallthrough__ attribute. [[fallthrough]] is also available in C++17
and the next C2x, but not everyone uses C++17 or C2x.

So define the SDL_FALLTHROUGH macro to deal with those problems - if we
are using C++17 or C2x, it expands to [[fallthrough]]; else if the
compiler has __has_attribute and has the __fallthrough__ attribute, then
it expands to __attribute__((__fallthrough__)); else it expands to an
empty statement, with a /* fallthrough */ comment (it's a do {} while
(0) statement, because users of this macro need to use a semicolon,
because [[fallthrough]] and __attribute__((__fallthrough__)) require a
semicolon).

Clang before Clang 10 and GCC before GCC 7 have problems with using
__attribute__ as a sole statement and warn about a "declaration not
declaring anything", so fall back to using the /* fallthrough */ comment
if we are using those older compiler versions.

Applications using SDL are also free to use this macro (because it is
defined in begin_code.h).

All existing /* fallthrough */ comments have been replaced with this
macro. Some of them were unnecessary because they were the last case in
a switch; using SDL_FALLTHROUGH in those cases would result in a compile
error on compilers that support __fallthrough__, for having a
__attribute__((__fallthrough__)) statement that didn't immediately
precede a case label.

src/video/directfb/SDL_DirectFB_WM.c

@@ -323,7 +323,7 @@ DirectFB_WM_ProcessEvent(_THIS, SDL_Window * window, DFBWindowEvent * evt)
                 }
                 if (window->flags & SDL_WINDOW_MAXIMIZED)
                     return 1;
-                /* fall through */
+                SDL_FALLTHROUGH;
             default:
                 windata->wm_grab = pos;
                 if (grabbed_window != NULL)

src/video/directfb/SDL_DirectFB_events.c

@@ -261,7 +261,7 @@ ProcessWindowEvent(_THIS, SDL_Window *sdlwin, DFBWindowEvent * evt)
                 SDL_SendWindowEvent(sdlwin, SDL_WINDOWEVENT_MOVED,
                                     evt->x, evt->y);
             }
-            /* fall throught */
+            SDL_FALLTHROUGH;
         case DWET_SIZE:
             /* FIXME: what about < 0 */
             evt->w -= (windata->theme.right_size + windata->theme.left_size);