CMakeLists: Specify -Wextra on linux builds

Allows reporting more cases where logic errors may exist, such as
implicit fallthrough cases, etc.

We currently ignore unused parameters, since we currently have many
cases where this is intentional (virtual interfaces).

While we're at it, we can also tidy up any existing code that causes
warnings. This also uncovered a few bugs as well.

src/input_common/sdl/sdl_impl.cpp

@@ -603,6 +603,7 @@ public:
                 if (std::abs(event.jaxis.value / 32767.0) < 0.5) {
                     break;
                 }
+                [[fallthrough]];
             case SDL_JOYBUTTONUP:
             case SDL_JOYHATMOTION:
                 return SDLEventToButtonParamPackage(state, event);