graphics/poppler: update to 21.04.0

Release 21.04.0:
        core:
         * Hide symbols by default
         * TextSelectionDumper: fix word order for RTL text
         * Fix rendering of text in some files. Issue #1052
         * Implement rendering of Masks of Image subtype. Issue #1058
         * Forms: fix unclicking standalone form buttons. Issue #1034

        glib:
         * Expose more fields from MediaRendition in PopplerMedia
         * Use stock glib macro to define boxed type
         * Remove incorrecly used volatile from enum type registration code

        qt5:
         * Fix crash in files with malformed signatures
         * Fix memory leak when QImage constructor "fails"

        qt6:
         * Fix crash in files with malformed signatures
         * Fix memory leak when QImage constructor "fails"

        utils:
         * pdfsig: New paragraph for "-sign" in manpage
         * pdfimages: Do not assert in "too big images". Issue #1061

        build system:
         * Require cmake >= 3.10
         * Require Qt 5 >= 5.9
         * Require glib >= 2.56
         * Require gtk 3 >= 3.22
         * Require gdk-pixbuf >= 2.36

PR:		poppler-21.04.0
Exp-run by:	antoine