[wktraster] Implement ST_AsJPEG function for raster

[mloskot]

This tasks belongs to the Objective 0.1.6f specified in the WKT Raster roadmap

As stated at the end of the comment 5 of the "Specification Comments following the Code Sprint" section of the working specifications, this function should be a PL/pgSQL wrapper around a more general ST_AsImage(format, params) function using GDAL to convert any raster to an image.

The function should return a bytea.

This function is particularly sensible to the number of band present in the raster as it normally needs 3 band (r,g,b).

It is also very sensible to the pixel type of the raster since JPEG supports only values in the 0-255 range.

See the working specifications for more details/questions.

Priority should be given to these variants:

ST_AsJPEG(raster, integer)
ST_AsJPEG(raster, integer, integer, integer, integer)
ST_AsJPEG(raster, integer, "GRAYSCALE", integer, min, max, text)