Declare data param of avifRWStreamWrite as void *

Declare the 'data' parameter of avifRWStreamWrite() as const void *.
This avoids the need to cast that argument to const uint8_t * at some
call sites.
diff --git a/include/avif/internal.h b/include/avif/internal.h
index 4e9c15e..4ebbc1e 100644
--- a/include/avif/internal.h
+++ b/include/avif/internal.h
@@ -213,7 +213,7 @@
 void avifRWStreamSetOffset(avifRWStream * stream, size_t offset);
 
 void avifRWStreamFinishWrite(avifRWStream * stream);
-void avifRWStreamWrite(avifRWStream * stream, const uint8_t * data, size_t size);
+void avifRWStreamWrite(avifRWStream * stream, const void * data, size_t size);
 void avifRWStreamWriteChars(avifRWStream * stream, const char * chars, size_t size);
 avifBoxMarker avifRWStreamWriteBox(avifRWStream * stream, const char * type, size_t contentSize);
 avifBoxMarker avifRWStreamWriteFullBox(avifRWStream * stream, const char * type, size_t contentSize, int version, uint32_t flags);