(Qcenter): Declare here.

(Qcenter): Declare here.
(syms_of_xdisp): intern and staticpro it.
(handle_single_display_prop): Allow space display property on all
platforms.
(display_mode_line): Set mode_line_p before displaying line.
(calc_pixel_width_or_height): Declare extern.  Add separate :align-to
handling.  Remove complex cases for fringes and scroll-bars.
Add left, right, and center alignment positions. Add text (area)
width/height.  Return width or height for image specs.
(produce_stretch_glyph): Improve handling of :align-to.  Is now
relative to left of text area by default, but other base offsets
can be specified -- also for text lines.