Add a BWF test.

This test verifies that the bext chunk is read in correctly.
Move some definitions more locally.
Fix a regression in reading in bext chunks.

Fix a regression in reading in bext chunks that I introduced in commit
b7e74101d7bf261661e76c430ee3f812a212b527.  The original code I called a
"*terrible* workaround" was actually very much necessary (and for the correct
reason).  This commit adds a VLA version of the original code.