Fix for avio_alloc_context on FFmpeg 7

Author: h4tr3d

src/formatcontext.cpp

@@ -21,7 +21,11 @@ int custom_io_read(void *opaque, uint8_t *buf, int buf_size)
     return io->read(buf, buf_size);
 }
 
+#if (LIBAVFORMAT_VERSION_MAJOR < 61)
 int custom_io_write(void *opaque, uint8_t *buf, int buf_size)
+#else
+int custom_io_write(void *opaque, const uint8_t *buf, int buf_size)
+#endif
 {
     if (!opaque)
         return -1;