Declare some stream functions to take const param

Declare some stream functions to take a const stream pointer parameter.
This helps code analysis.

src/stream.c

diff --git a/src/stream.c b/src/stream.c
index 30402a5..3270061 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -20,17 +20,17 @@
     stream->offset = 0;
 }
 
-avifBool avifROStreamHasBytesLeft(avifROStream * stream, size_t byteCount)
+avifBool avifROStreamHasBytesLeft(const avifROStream * stream, size_t byteCount)
 {
     return (stream->offset + byteCount) <= stream->raw->size;
 }
 
-size_t avifROStreamRemainingBytes(avifROStream * stream)
+size_t avifROStreamRemainingBytes(const avifROStream * stream)
 {
     return stream->raw->size - stream->offset;
 }
 
-size_t avifROStreamOffset(avifROStream * stream)
+size_t avifROStreamOffset(const avifROStream * stream)
 {
     return stream->offset;
 }
@@ -215,7 +215,7 @@
     stream->offset = 0;
 }
 
-size_t avifRWStreamOffset(avifRWStream * stream)
+size_t avifRWStreamOffset(const avifRWStream * stream)
 {
     return stream->offset;
 }

src/write.c

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);