(struct glyph_row): New member extra_line_spacing.

(struct glyph_row): New member extra_line_spacing.
(struct it): New member max_extra_line_spacing.
(MR_PARTIALLY_VISIBLE, MR_PARTIALLY_VISIBLE_AT_TOP)
(MR_PARTIALLY_VISIBLE_AT_BOTTOM): New helper macros.
(MATRIX_ROW_PARTIALLY_VISIBLE_P): Fix to return false if invisible
part of last line is only extra line spacing (so the text on the
line is fully visible). Use helper macros.
Add W arg (to use them).  All callers changed.
(MATRIX_ROW_PARTIALLY_VISIBLE_AT_TOP_P)
(MATRIX_ROW_PARTIALLY_VISIBLE_AT_BOTTOM_P): Use helper macros.