Improve pixel-scroll-mode

Scroll vertically by number of pixels returned by
'frame-char-height' with or without horizontally scrolled.
(Bug#28922)

* lisp/pixel-scroll.el (pixel-resolution-fine-flag): When t, scroll
by number of pixels returned by 'frame-char-height'.
(pixel-scroll-up): Scroll by 'frame-char-height'. Fix algorithm to
move cursor to avoid unexpected jump.
(pixel-scroll-down): Scroll by 'frame-char-height'.
(pixel-bob-at-top-p): Consider number of pixels that is about to
scroll.
(pixel-posn-y-at-point): Consider existence of an overlay string.
Return nil when horizontally scrolled.
(pixel-point-at-top-p): Consider number of pixels that is about to
scroll. Use different algorithm when horizontally scrolled.
(pixel-point-at-bottom-p): Consider number of pixels that is about
to scroll. Return nil when horizontally scrolled.
(pixel-scroll-pixel-down): Move cursor when horizontally scrolled.
(pixel--whistlestop-line-up): Change cosmetics and move cursor when
horizontally scrolled.
(pixel-line-height): Call 'pixel-visual-line-height' instead of
'line-pixel-height'.
(pixel-visual-line-height): New function to return height in pixels
of text line where cursor is with or without horizontally scrolled,
considering response of display engine.
(pixel-visible-pos-in-window): New function to return position of
a char shown on text line where cursor is on screen with or without
horizontally scrolled.

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