(Vdisplay_pixels_per_inch): New variable.

(Vdisplay_pixels_per_inch): New variable.
(Vshow_text_cursor_in_void): New variable.
(glyph_to_pixel_coords): Don't use negative hpos.
(x_y_to_hpos_vpos): Fix for partially visible first glyph.
(append_stretch_glyph): Change ascent arg to be actual value
in pixels rather than ratio to height. Callers changed.
(calc_pixel_width_or_height): New aux function, implementing
pixel based artihmetic for glyph widths and heights.
(produce_stretch_glyph): Use calc_pixel_width_or_height for
:width, :height, :align-to, and :ascent, thus allowing these to
be specified in pixels as well as multiples of characters.
Don't produce stretch glyphs with zero width or height.
(get_specified_cursor_type): Declare static.
(get_window_cursor_type): Declare static.  Add glyph arg to be
able to know when cursor is on an image; always substitute
hollow-box cursor for filled-box cursor on images, to avoid
negative images and flicker when blinking the cursor.
(display_and_set_cursor): Pass glyph to	get_window_cursor_type.
(note_mode_line_or_margin_highlight): Use non-text cursor rather
than vertical scroll-bar cursor in display margins.
(note_mouse_highlight): Use non-text cursor rather than text
cursor in fringes and over images in the text area.
Use non-text cursor when mouse pointer is outside editable text,
i.e. in the void after end-of-line or end-of-buffer; this was
already done for W32, but is now standard for all systems --
user can toggle show-text-cursor-in-void to get old behaviour.
(syms_of_xdisp): DEFVAR_LISP Vshow_text_cursor_in_void and
Vdisplay_pixels_per_inch.