Make scrcpy.h independant of other headers

The header scrcpy.h is intended to be the "public" API. It should not
depend on other internal headers.

Therefore, declare all required structs in this header and adapt
internal code.

app/src/scrcpy.c

@@ -8,6 +8,8 @@
 #include <SDL2/SDL.h>
 
 #ifdef _WIN32
+// not needed here, but winsock2.h must never be included AFTER windows.h
+# include <winsock2.h>
 # include <windows.h>
 #endif