Use "always on top" only for SDL >= 2.0.5

The flag SDL_WINDOW_ALWAYS_ON_TOP is available since SDL 2.0.5.

Do not use it if SDL is older, to fix compilation failure.

Fixes <https://github.com/Genymobile/scrcpy/issues/432>

app/src/compat.h

@@ -39,6 +39,8 @@
 # define SCRCPY_SDL_HAS_HINT_MOUSE_FOCUS_CLICKTHROUGH
 // <https://wiki.libsdl.org/SDL_GetDisplayUsableBounds>
 # define SCRCPY_SDL_HAS_GET_DISPLAY_USABLE_BOUNDS
+// <https://wiki.libsdl.org/SDL_WindowFlags>
+# define SCRCPY_SDL_HAS_WINDOW_ALWAYS_ON_TOP
 #endif
 
 #endif

app/src/screen.c

@@ -157,7 +157,12 @@ SDL_bool screen_init_rendering(struct screen *screen,
     window_flags |= SDL_WINDOW_ALLOW_HIGHDPI;
 #endif
     if (always_on_top) {
+#ifdef SCRCPY_SDL_HAS_WINDOW_ALWAYS_ON_TOP
         window_flags |= SDL_WINDOW_ALWAYS_ON_TOP;
+#else
+        LOGW("The 'always on top' flag is not available "
+             "(compile with SDL >= 2.0.5 to enable it)");
+#endif
     }
 
     screen->window = SDL_CreateWindow(device_name, SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,