LibPDF+LibGfx: Pass jbig2-filtered data to JBIG2ImageDecoderPlugin

Except for /JBIG2Globals, which we bail out on for now. In my 1000
files, 13 use JBIG2, and of those, 2 use JBIG2Globals (0000372.pdf e.g.
page 11 and 0000857.pdf e.g. page 1), and only one (the latter) of the
two uses the same JBIG2Globals stream for more than a single image.

JBIG2ImageDecoderPlugin cannot decode the data yet, so no behavior
change, but with `#define JBIG2_DEBUG 1` at the top of that file,
it now prints segment header info for PDFs containing JBIG2 data :^)

Userland/Libraries/LibPDF/CommonNames.h

@@ -112,6 +112,7 @@
     X(Info)                       \
     X(Intent)                     \
     X(JBIG2Decode)                \
+    X(JBIG2Globals)               \
     X(JPXDecode)                  \
     X(K)                          \
     X(Keywords)                   \