codecs: m4a: improve seek accuracy

Seeking doesn't work well in M4A files with very few chunks due to
the seek method used (chunk based using the info in the 'stco' atom).
According to libm4a/demux.c the expected seek resolution using this
method is 1/4 to 1/2 seconds. However, ffmpeg generates files with a
1 megabyte chunk size, so the resolution is much worse than expected
on some files: around 30-40 seconds at 256kbps.

There was a bug with the seek position reported back to Rockbox: the
codec pretended it could seek exactly to the requested sample, but it
would only seek to the start of a chunk. This could leave the UI in a
confusing state because the real playback position was different from
what the elapsed time showed. Fix this by recalculating the reported
sample position using the chunk start.

To fix the low seek accuracy, use the table in the 'stsz' atom to skip
individual packets within a chunk. This is very accurate, but it takes
a lot of RAM to allocate the table. Currently the table is not allowed
to use more than half of the codec RAM, which should suffice for short
files on most targets. On files where the table is too large the codec
will fall back to the less accurate chunk-based seek method.

Change-Id: Ide38ea846c1cdd69691e9b1e1cd87eb0fa11cf78

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