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/render/software/SDL_draw.h

@@ -618,10 +618,10 @@ do { \
     while (height--) { \
         { int n = (width+3)/4; \
             switch (width & 3) { \
-            case 0: do {   op; pixel++; /* fallthrough */ \
-            case 3:        op; pixel++; /* fallthrough */ \
-            case 2:        op; pixel++; /* fallthrough */ \
-            case 1:        op; pixel++; /* fallthrough */ \
+            case 0: do {   op; pixel++; SDL_FALLTHROUGH; \
+            case 3:        op; pixel++; SDL_FALLTHROUGH; \
+            case 2:        op; pixel++; SDL_FALLTHROUGH; \
+            case 1:        op; pixel++; \
                     } while ( --n > 0 ); \
             } \
         } \