Page overlap: add "Horizontal line" marker

[hius07]

Closes #10394.

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This change is 

[poire-z]

Or instead of 2, one of (or a new among them) the Size.line.medium/thick ?

[hius07]

We have this "2" in highlighting drawer and marker too:

koreader/frontend/apps/reader/modules/readerview.lua

Lines 614 to 632 in 2ba694d

	function ReaderView:drawHighlightRect(bb, _x, _y, rect, drawer, draw_note_mark)
	local x, y, w, h = rect.x, rect.y, rect.w, rect.h
	if drawer == "lighten" then
	bb:lightenRect(x, y, w, h, self.highlight.lighten_factor)
	elseif drawer == "underscore" then
	bb:paintRect(x, y + h - 1, w, 2, Blitbuffer.COLOR_GRAY)
	elseif drawer == "strikeout" then
	local line_y = y + math.floor(h / 2) + 1
	if self.ui.paging then
	line_y = line_y + 2
	end
	bb:paintRect(x, line_y, w, 2, Blitbuffer.COLOR_BLACK)
	elseif drawer == "invert" then
	bb:invertRect(x, y, w, h)
	end
	if draw_note_mark then
	if self.highlight.note_mark == "underline" then
	bb:paintRect(x, y + h - 1, w, 2, Blitbuffer.COLOR_BLACK)
	else

[Frenzie]

That probably just means we overlooked it at the time. This does look like a Size.line.medium situation. :-)