Bring pgtk more inline with X11-cairo builds

Bring pgtk more inline with X11-cairo builds

* src/pgtkterm.h (struct pgtk_output):

* src/pgtkterm.c (FRAME_CR_ACTIVE_CONTEXT, flip_cr_context)
(x_draw_bar_cursor, pgtk_scroll_run, pgtk_update_end)
(pgtk_cr_draw_image, pgtk_flash, pgtk_handle_draw)
(pgtk_cr_update_surface_desired_size, pgtk_begin_cr_clip)
(pgtk_cr_destroy_frame_context):

* src/image.c (image_create_pattern_from_pixbuf):

- Don't create a context each time, just hold a reference

- Pull in a few cairo image changes from master

- Remove redundant code in pgtk_update_begin()