Support svgz in graphicfilter (import image to document)

Detect if input stream is gzip, if it is decompress and check the
decompressed stream if the content is svg.
When reading, decompress the whole svg content first, and then
store it internally.

Change-Id: Ie8b7e2b2f59a92d6420c38fd4a25637a6ee94425
fdo#75487 Register svgz as a recognisable extension.

Change-Id: If4b5d6a38b468e5412dd973547cbcd580a83e239
fdo#75487 Add support for svgz to SvgFilter and co.

Draw can open a svg file directly as a "document" not only as a
picture (which is handled by "graphicfilter" in vcl). This commit
adds support to also open svgz files by decompressing the stream
first.

Change-Id: I241e695076b380735dc9b9fc27f5416fff68f6ab