Add support for x-support-frames

[fejfighter]

• src/pgtkfns.c (Fx_export_frames): (syms_of_pgtkfns): port from X11 version
• src/pgtkterm.c (pgtk_cr_accumulate_data): (pgtk_cr_destroy): (pgtk_cr_export_frames): port from X11
• src/pgtkterm.h: add defs

[fejfighter]

I saw this post on reddit: https://www.reddit.com/r/emacs/comments/idz35e/emacs_27_can_take_svg_screenshots_of_itself/

This should add that support

[masm11]

thanks.