core: Correctly calculate the length passed to `resize_to_reader`

In several places, we read some data from a tag, and then pass
the original tag length to `resize_to_reader`. This is incorrect -
the provided length is used an an offset from the current position
in the reader, so we will extend past the end of the current tag if
we've already read some bytes.

In practice, this doesn't appear to cause any problems - AVM bytecode
has internal length fields, which end up ensuring that we will never
try to read past where the slice *should* end. However, if a `DoAbc`
tag is the last tag in the file, then we'll end up trying to use
`resize_to_reader` with an offset past the end of the movie.

This commit subtracts the number of already-read bytes from `tag_len`,
to ensure that we always construct a correctly-sized `SwfSlice`

core/src/display_object/movie_clip.rs

@@ -464,16 +464,19 @@ impl<'gc> MovieClip<'gc> {
             return Ok(());
         }
 
+        let start = reader.as_slice();
         // Queue the init actions.
         // TODO: Init actions are supposed to be executed once, and it gives a
         // sprite ID... how does that work?
         let _sprite_id = reader.read_u16()?;
+        let num_read = reader.pos(start);
+
         let slice = self
             .0
             .read()
             .static_data
             .swf
-            .resize_to_reader(reader, tag_len)
+            .resize_to_reader(reader, tag_len - num_read)
             .ok_or_else(|| {
                 std::io::Error::new(
                     std::io::ErrorKind::Other,
@@ -499,6 +502,7 @@ impl<'gc> MovieClip<'gc> {
             return Ok(());
         }
 
+        let start = reader.as_slice();
         // Queue the actions.
         // TODO: The tag reader parses the entire ABC file, instead of just
         // giving us a `SwfSlice` for later parsing, so we have to replcate the
@@ -507,6 +511,7 @@ impl<'gc> MovieClip<'gc> {
         let name = reader.read_str()?.to_string_lossy(reader.encoding());
         let is_lazy_initialize = flags & 1 != 0;
         let domain = context.library.library_for_movie_mut(movie).avm2_domain();
+        let num_read = reader.pos(start);
 
         // The rest of the tag is an ABC file so we can take our SwfSlice now.
         let slice = self
@@ -514,7 +519,7 @@ impl<'gc> MovieClip<'gc> {
             .read()
             .static_data
             .swf
-            .resize_to_reader(reader, tag_len)
+            .resize_to_reader(reader, tag_len - num_read)
             .ok_or_else(|| {
                 std::io::Error::new(
                     std::io::ErrorKind::Other,
@@ -2891,14 +2896,17 @@ impl<'gc, 'a> MovieClipData<'gc> {
         reader: &mut SwfStream<'a>,
         tag_len: usize,
     ) -> DecodeResult {
+        let start = reader.as_slice();
         let id = reader.read_character_id()?;
         let num_frames = reader.read_u16()?;
+        let num_read = reader.pos(start);
+
         let movie_clip = MovieClip::new_with_data(
             context.gc_context,
             id,
             self.static_data
                 .swf
-                .resize_to_reader(reader, tag_len - 4)
+                .resize_to_reader(reader, tag_len - num_read)
                 .ok_or_else(|| {
                     std::io::Error::new(
                         std::io::ErrorKind::Other,