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.

test/controllermap.c

@@ -562,7 +562,7 @@ WatchJoystick(SDL_Joystick * joystick)
                 if ((event.key.keysym.sym != SDLK_ESCAPE)) {
                     break;
                 }
-                /* Fall through to signal quit */
+                SDL_FALLTHROUGH;
             case SDL_QUIT:
                 done = SDL_TRUE;
                 break;
@@ -755,7 +755,7 @@ main(int argc, char *argv[])
                 if ((event.key.keysym.sym != SDLK_ESCAPE)) {
                     break;
                 }
-                /* Fall through to signal quit */
+                SDL_FALLTHROUGH;
             case SDL_QUIT:
                 done = SDL_TRUE;
                 break;

test/testgamecontroller.c

@@ -394,7 +394,7 @@ loop(void *arg)
             if (event.key.keysym.sym != SDLK_ESCAPE) {
                 break;
             }
-            /* Fall through to signal quit */
+            SDL_FALLTHROUGH;
         case SDL_QUIT:
             done = SDL_TRUE;
             break;

test/testjoystick.c

@@ -186,7 +186,7 @@ loop(void *arg)
                 (event.key.keysym.sym != SDLK_AC_BACK)) {
                 break;
             }
-            /* Fall through to signal quit */
+            SDL_FALLTHROUGH;
         case SDL_FINGERDOWN:
         case SDL_MOUSEBUTTONDOWN:
         case SDL_QUIT: