Consolidate gui-independent "glyph string" code here.

Consolidate gui-independent "glyph string" code here.
(dump_glyph_string): Moved here.
(init_glyph_string, append_glyph_string_lists, append_glyph_string)
(prepend_glyph_string_lists, get_glyph_face_and_encoding)
(fill_composite_glyph_string, fill_glyph_string)
(fill_image_glyph_string, fill_stretch_glyph_string)
(left_overwritten, left_overwriting, right_overwritten)
(right_overwriting, get_char_face_and_encoding)
(set_glyph_string_background_width, compute_overhangs_and_x)
(append_glyph, append_composite_glyph, produce_image_glyph)
(take_vertical_position_into_account, append_stretch_glyph)
(produce_stretch_glyph): New generic functions (based on X version).
Call platform specific functions through rif.
(INIT_GLYPH_STRING): New macro, hides W32 details.
(BUILD_STRETCH_GLYPH_STRING, BUILD_IMAGE_GLYPH_STRING)
(BUILD_CHAR_GLYPH_STRINGS, BUILD_COMPOSITE_GLYPH_STRING)
(BUILD_GLYPH_STRINGS): Generic macros (based on X version).
(x_draw_glyphs, x_get_glyph_overhangs, x_produce_glyphs)
(notice_overwritten_cursor):
Generic functions exported to platform modules.  Users changed.