Output format: add a AGG/MIXED pseudo driver.

[rouault]

Implementing https://lists.osgeo.org/pipermail/mapserver-dev/2016-June/thread.html#14878

This driver redirects to a concrete driver when the image
has non-opaque pixels, and to another one when the image
is fully opaque.

Requires to define TRANSPARENT_FORMAT and OPAQUE_FORMAT
FORMAT_OPTION on OUTPUTFORMATs that instanciate that
driver. Those underlying formats can be AGG/PNG, AGG/JPEG
or a GDAL/ one (anyone that uses AGG/ for rendering)

Two predefined formats image/jpeg-png and image/jpeg-png8
are added.

Example of custom mixed formats:

OUTPUTFORMAT
  NAME jpeg_low
  MIMETYPE "image/jpeg; quality=50"
  DRIVER AGG/JPEG
  IMAGEMODE RGB
  FORMATOPTION "QUALITY=50"
END

OUTPUTFORMAT
  NAME jpeg_low_png
  MIMETYPE "image/jpeg_low-png"
  DRIVER AGG/MIXED
  FORMATOPTION "TRANSPARENT_FORMAT=png"
  FORMATOPTION "OPAQUE_FORMAT=jpeg_low"
END

OUTPUTFORMAT
  NAME jpeg_high
  MIMETYPE "image/jpeg; quality=99"
  DRIVER AGG/JPEG
  IMAGEMODE RGB
  FORMATOPTION "QUALITY=99"
END

OUTPUTFORMAT
  NAME jpeg_high_png8
  MIMETYPE "image/jpeg_high-png8"
  DRIVER AGG/MIXED
  FORMATOPTION "TRANSPARENT_FORMAT=png8"
  FORMATOPTION "OPAQUE_FORMAT=jpeg_high"
END

[tbonfort]

LGTM
👍 once the tests are updated to reference the new default mimetypes

[rouault]

Oh, another thing is the naming "AGG/MIXED" which doesn't reflect that GDAL drivers can actually be used as TRANSPARENT_FORMAT or OPAQUE_FORMAT. But perhaps this can be adressed by a note in the documentation.