avcodec/cbs: Factor out common code for writing units

All cbs-functions to write units share a common pattern:
1. They check whether they have a write buffer (that is used to store
the unit's data until the needed size becomes known after writing the
unit when a dedicated buffer will be allocated).
2. They use this buffer for a PutBitContext.
3. The (codec-specific) writing takes place through the PutBitContext.
4. The return value is checked. AVERROR(ENOSPC) here always indicates
that the buffer was too small and leads to a reallocation of said
buffer.
5. The final buffer will be allocated and the data copied.

This commit factors this common code out in a single function in cbs.c.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 7c92eaace2)

libavcodec/cbs_jpeg.h

@@ -120,11 +120,4 @@ typedef struct JPEGRawComment {
 } JPEGRawComment;
 
 
-typedef struct CodedBitstreamJPEGContext {
-    // Write buffer.
-    uint8_t *write_buffer;
-    size_t write_buffer_size;
-} CodedBitstreamJPEGContext;
-
-
 #endif /* AVCODEC_CBS_JPEG_H */