enable panning over zoomed page

This works only in ZOOM_BY_VALUE mode invoked by Shift+Page< or Shift+Page>

Panning is modal: in normal mode, you move using fiveway but
when you press fiveway you switch to move-by-screen mode
(which is useful after you centered first column of article
and you just want to move around)

At any time you can press fiveway with shift to move in even smaller
increments

reader.lua

@@ -69,6 +69,10 @@ fullheight = 0
 offset_x = 0
 offset_y = 0
 
+shift_x = 100
+shift_y = 50
+shift_step = 1 -- using shift_[xy] or width/height
+
 shiftmode = false
 
 if optarg["d"] == "k3" then
@@ -156,9 +160,9 @@ end
 function show(no)
 	local slot
 	if globalzoommode ~= ZOOM_BY_VALUE then
-		slot = draworcache(no,globalzoommode,0,0,width,height,globalgamma)
+		slot = draworcache(no,globalzoommode,offset_x,offset_y,width,height,globalgamma)
 	else
-		slot = draworcache(no,globalzoom,0,0,width,height,globalgamma)
+		slot = draworcache(no,globalzoom,offset_x,offset_y,width,height,globalgamma)
 	end
 	fb:blitFullFrom(cache[slot].bb)
 	if rcount == rcountmax then
@@ -182,9 +186,9 @@ function goto(no)
 	if no < doc:getPages() then
 		-- always pre-cache next page
 		if globalzoommode ~= ZOOM_BY_VALUE then
-			draworcache(no,globalzoommode,0,0,width,height,globalgamma)
+			draworcache(no,globalzoommode,offset_x,offset_y,width,height,globalgamma)
 		else
-			draworcache(no,globalzoom,0,0,width,height,globalgamma)
+			draworcache(no,globalzoom,offset_x,offset_y,width,height,globalgamma)
 		end
 	end
 end
@@ -252,6 +256,47 @@ function mainloop()
 					setglobalzoommode(ZOOM_FIT_TO_PAGE_HEIGHT)
 				end
 			end
+
+			if globalzoommode == ZOOM_BY_VALUE then
+				local x
+				local y
+
+				if shiftmode then -- shift always moves in small steps
+					x = shift_x / 3
+					y = shift_y / 3
+				elseif shift_step then
+					x = shift_x
+					y = shift_y
+				else
+					x = width  - 5; -- small overlap when moving by page
+					y = height - 5;
+				end
+
+				print("offset "..offset_x.."*"..offset_x.." shift "..x.."*"..y.." globalzoom="..globalzoom)
+
+				if ev.code == KEY_FW_LEFT then
+					offset_x = offset_x + x
+					goto(pageno)
+				elseif ev.code == KEY_FW_RIGHT then
+					offset_x = offset_x - x
+					goto(pageno)
+				elseif ev.code == KEY_FW_UP then
+					offset_y = offset_y - y
+					goto(pageno)
+				elseif ev.code == KEY_FW_DOWN then
+					offset_y = offset_y + y
+					goto(pageno)
+				elseif ev.code == KEY_FW_PRESS then
+					if shiftmode then
+						offset_x = 0
+						offset_y = 0
+						goto(pageno)
+					else
+						shift_step = not shift_step
+					end
+				end
+			end
+
 			local nsecs, nusecs = util.gettime()
 			local dur = (nsecs - secs) * 1000000 + nusecs - usecs
 			print("E: T="..ev.type.." V="..ev.value.." C="..ev.code.." DUR="..dur)