Declare some stream functions to take const param

Declare some stream functions to take a const stream pointer parameter.
This helps code analysis.
diff --git a/src/write.c b/src/write.c
index 4771ae0..d804be3 100644
--- a/src/write.c
+++ b/src/write.c
@@ -155,7 +155,7 @@
     avifFree(data);
 }
 
-static void avifEncoderItemAddMdatFixup(avifEncoderItem * item, avifRWStream * s)
+static void avifEncoderItemAddMdatFixup(avifEncoderItem * item, const avifRWStream * s)
 {
     avifOffsetFixup * fixup = (avifOffsetFixup *)avifArrayPushPtr(&item->mdatFixups);
     fixup->offset = avifRWStreamOffset(s);