Using instance in repeat-until to peek the next X bits
Hey, I have the following usecase. In mpeg2 streams the start indicator is a 24 bit 1 value. I have to read the stream byte aligned so I though I can do something like this:
```
meta:
  id: mpeg2_video
  file-extension: mp2
  endian: le
seq:
  - id: prefix
    type: u1
    repeat: until
    repeat-until: prefix_code != 1
  - id: ignore
    type: b24
instances:
  prefix_code:
    io: _root._io
    pos: _root._io.pos
    size: b24
```
this does not seem to work as I get an error during compile (to python):
`mpeg2_video: /seq/0/repeat-until: can't compare BytesLimitType(Name(identifier(b24)),None,false,None,None) and Int1Type(true)` probably I do something wrong, but I saw now other way to peek the coming bits/bytes. I tried to do `.to_i` on prefix_code but it seems BytesLimitType has no methods. Is there any way to achieve this currently? 