(init_iterator): Handle line-spacing float value.

(init_iterator): Handle line-spacing float value.
Initialize override_ascent member.
(append_space_for_newline): Reset override_ascent.
Remove use_default_face.
(calc_line_height_property): New function to calculate value of
line-height and line-spacing properties.  Look at overlays, too.
Set override_ascent, override_descent, override_boff members when
using another face than the current face.  Float values are now
relative to the frame default font, by default; accept a cons
of ratio and face name to specify value relative to a specific face.
(x_produce_glyphs): Use calc_line_height_property.
Use override_ascent etc. when set to handle different face heights.
A negative line-spacing property value is interpreted as a total
line height, rather than inter-line spacing.
(note_mouse_highlight): Allocate room for 40 overlays initially.