Fix padding size in Event

According to SDL_events.h, the SDL_Event union has 128 bytes of padding,
but our Event has 128 bits.

https://github.com/libsdl-org/SDL/blob/8eb57c5a420426f7b6d96f9c6afd0777c6137be6/include/SDL3/SDL_events.h#L1040

Author: coat

src/sdl3.zig

@@ -1771,7 +1771,7 @@ pub const Event = extern union {
     render: RenderEvent,
     drop: DropEvent,
     clipboard: ClipboardEvent,
-    _: u128, // padding
+    _: [128]u8, // padding
 };
 
 /// Pump the event loop, gathering events from the input devices.