Fix bug #14771 with scroll-step = 1 and non-nil line-spacing.

Fix bug #14771 with scroll-step = 1 and non-nil line-spacing.

 src/xdisp.c (default_line_pixel_height): New function.
 (pos_visible_p, move_it_vertically_backward, try_scrolling)
 (try_cursor_movement, redisplay_window, try_window)
 (try_window_id): Use it instead of FRAME_LINE_HEIGHT.  (Bug#14771)
 src/window.c (window_scroll_pixel_based): use
 default_line_pixel_height.
 src/dispextern.h (default_line_pixel_height): Add prototype.
 src/frame.c (x_set_line_spacing): Accept a float value for
 line-spacing parameter, per the documentation.