Fix stipple support on PGTK

Fix stipple support on PGTK

* src/image.c (image_create_pattern_from_pixbuf): Remove function.
(image_bitmap_to_cr_pattern): New function.
(image_create_bitmap_from_data):
(image_create_bitmap_from_file): Create patterns with the correct
mask values directly from bitmap data.  Also, don't rely on GDK
to read XPMs, as that format isn't supported anymore.
* src/pgtkterm.c (pgtk_bitmap_icon): Disable already nonfunctional
code.
(pgtk_display_x_warning): Wrap warning message.
* src/pgtkterm.h (struct pgtk_bitmap_record): Remove `img' field.
(bug#64969)