Fix -Wimplicit-fallthrough warnings from GCC 8

Adds `FALLTHROUGH` macro to specify when a fallthrough is intentional.
Can be replaced by `[[fallthrough]]` if/when we switch to C++17.

The warning is now enabled by default for GCC on `extra` warnings level
(part of GCC's `-Wextra`). It's not enabled in Clang's `-Wextra` yet,
but we could enable it manually once we switch to C++11. There's no
equivalent feature in MSVC for now.

Fixes #26135.

(cherry picked from commit fc370b3feb)

core/io/multiplayer_api.cpp

@@ -46,7 +46,8 @@ _FORCE_INLINE_ bool _should_call_local(MultiplayerAPI::RPCMode mode, bool is_mas
 		case MultiplayerAPI::RPC_MODE_MASTERSYNC: {
 			if (is_master)
 				r_skip_rpc = true; // I am the master, so skip remote call.
-		} // Do not break, fall over to other sync.
+			FALLTHROUGH;
+		}
 		case MultiplayerAPI::RPC_MODE_REMOTESYNC:
 		case MultiplayerAPI::RPC_MODE_PUPPETSYNC: {
 			// Call it, sync always results in a local call.

core/typedefs.h

@@ -331,7 +331,28 @@ struct _GlobalLock {
 /** This is needed due to a strange OpenGL API that expects a pointer
  *  type for an argument that is actually an offset.
  */
-
 #define CAST_INT_TO_UCHAR_PTR(ptr) ((uint8_t *)(uintptr_t)(ptr))
 
+/** Hint for compilers that this fallthrough in a switch is intentional.
+ *  Can be replaced by [[fallthrough]] annotation if we move to C++17.
+ *  Including conditional support for it for people who set -std=c++17
+ *  themselves.
+ *  Requires a trailing semicolon when used.
+ */
+#if __cplusplus >= 201703L
+#define FALLTHROUGH [[fallthrough]]
+#elif defined(__GNUC__) && __GNUC__ >= 7
+#define FALLTHROUGH __attribute__((fallthrough))
+#elif defined(__llvm__) && __cplusplus >= 201103L && defined(__has_feature)
+#if __has_feature(cxx_attributes) && defined(__has_warning)
+#if __has_warning("-Wimplicit-fallthrough")
+#define FALLTHROUGH [[clang::fallthrough]]
+#endif
+#endif
+#endif
+
+#ifndef FALLTHROUGH
+#define FALLTHROUGH
+#endif
+
 #endif // TYPEDEFS_H