convert.c is unmaintainable

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Originally reported on Google Code with ID 264

Just look at convert.c to be convinced. For example have a look at issue #171 and in
particular the patch needed to clamp value returned from openjpeg. All formats are
affected by the issue.

This means that we could have our test suite be valid for PGX output, but invalid for
other, this feel really unconfortable.

Reported by malaterre on 2014-02-28 08:38:42

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I think convert.c should support pgx, raw, tiff and png. Its used only for test suite
and give example through applications.

Reported by savmickael on 2014-03-06 11:54:46

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I think that convert.c needs to be split.
IMHO, there shall be one file per image format with one helper file which deals with
component bit depth limitations, component transform (e.g. from transform one line
from opj_image to 8bpp interleaved RGB with clamping etc...)

We can see that all imagetoxxx share a great deal of code. The minimum effort needed
is to get this code merged in helper functions that are used consistently throughout
those imagetoxxx functions.

Reported by mayeut on 2014-10-16 22:05:50

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I started to split it starting at r2972 to get Issue 203 a bump.

Reported by mayeut on 2014-12-20 18:51:32