Disable scrolling optimizations when a stipple is present

Disable scrolling optimizations when a stipple is present

* src/dispextern.h (struct glyph_row): New field `stippled_p'.
We cannot just use the contents of the glyph row, since it has
to be set in `gui_clear_end_of_line' and is more convenient to
set inside the various draw_glyph_string functions.

* src/dispnew.c (scrolling_window): Disable if a row in the
current matrix has the stipple_p flag set.

* src/xdisp.c (gui_clear_end_of_line):
* src/xterm.c (x_draw_image_glyph_string)
(x_draw_stretch_glyph_string, x_draw_glyph_string): Set
`stipple_p' if a stipple pattern was drawn.