PDF free zoom mode revisit

this should implement feature request of zoom mode for multi-columns
page in #501
This PR depends on koreader/koreader-base#435

How to use?
1. Tap the top left corner of a PDF/Djvu page to get into the flipping
mode
2. Double-tap on text block will zoom in to that column
3. Double-tap on any area will zoom out to an overview of the page
4. repeat step 2 to focus to another page block

How does it work?
1. We first find the mask of text blocks in the page. (Pic 1)
2. Then we intersect page boxes with user tap to form a page block. (Pic 2)
3. Finally we zoom the page to the page block and center current view to
that block. (Pic 3)

frontend/apps/reader/modules/readerzooming.lua

@@ -113,7 +113,7 @@ function ReaderZooming:onReadSettings(config)
 end
 
 function ReaderZooming:onSaveSettings()
-    self.ui.doc_settings:saveSetting("zoom_mode", self.zoom_mode)
+    self.ui.doc_settings:saveSetting("zoom_mode", self.orig_zoom_mode or self.zoom_mode)
 end
 
 function ReaderZooming:onSpread(arg, ges)
@@ -141,7 +141,6 @@ end
 function ReaderZooming:onToggleFreeZoom(arg, ges)
     if self.zoom_mode ~= "free" then
         self.orig_zoom = self.zoom
-        self.orig_zoom_mode = self.zoom_mode
         local xpos, ypos
         self.zoom, xpos, ypos = self:getRegionalZoomCenter(self.current_page, ges.pos)
         DEBUG("zoom center", self.zoom, xpos, ypos)
@@ -152,7 +151,7 @@ function ReaderZooming:onToggleFreeZoom(arg, ges)
         end
         self.view:SetZoomCenter(xpos, ypos)
     else
-        self.ui:handleEvent(Event:new("SetZoomMode", self.orig_zoom_mode or "page"))
+        self.ui:handleEvent(Event:new("SetZoomMode", "page"))
     end
 end
 
@@ -207,6 +206,20 @@ function ReaderZooming:onReZoom()
     return true
 end
 
+function ReaderZooming:onEnterFlippingMode(zoom_mode)
+    self.orig_zoom_mode = self.zoom_mode
+    if zoom_mode == "free" then
+        self.ui:handleEvent(Event:new("SetZoomMode", "page"))
+    else
+        self.ui:handleEvent(Event:new("SetZoomMode", zoom_mode))
+    end
+end
+
+function ReaderZooming:onExitFlippingMode(zoom_mode)
+    self.orig_zoom_mode = nil
+    self.ui:handleEvent(Event:new("SetZoomMode", zoom_mode))
+end
+
 function ReaderZooming:getZoom(pageno)
     -- check if we're in bbox mode and work on bbox if that's the case
     local zoom = nil
@@ -254,22 +267,19 @@ end
 function ReaderZooming:getRegionalZoomCenter(pageno, pos)
     local p_pos = self.view:getSinglePagePosition(pos)
     local page_size = self.ui.document:getNativePageDimensions(pageno)
-    local pos_x = p_pos.x / page_size.w / p_pos.zoom
-    local pos_y = p_pos.y / page_size.h / p_pos.zoom
-    local regions = self.ui.document:getPageRegions(pageno)
-    DEBUG("get page regions", regions)
+    local pos_x = p_pos.x / page_size.w
+    local pos_y = p_pos.y / page_size.h
+    local block = self.ui.document:getPageBlock(pageno, pos_x, pos_y)
     local margin = self.ui.document.configurable.page_margin * Screen:getDPI()
-    for i = 1, #regions do
-        if regions[i].x0 <= pos_x and pos_x <= regions[i].x1
-            and regions[i].y0 <= pos_y and pos_y <= regions[i].y1 then
-            local zoom = 1/(regions[i].x1 - regions[i].x0)
-            zoom = zoom/(1 + 3*margin/zoom/page_size.w)
-            local xpos = (regions[i].x0 + regions[i].x1)/2 * zoom * page_size.w
-            local ypos = p_pos.y / p_pos.zoom * zoom
-            return zoom, xpos, ypos
-        end
+    if block then
+        local zoom = self.dimen.w / page_size.w / (block.x1 - block.x0)
+        zoom = zoom/(1 + 3*margin/zoom/page_size.w)
+        local xpos = (block.x0 + block.x1)/2 * zoom * page_size.w
+        local ypos = p_pos.y / p_pos.zoom * zoom
+        return zoom, xpos, ypos
     end
-    return 2
+    local zoom = 2*self.dimen.w / page_size.w
+    return zoom/(1 + 3*margin/zoom/page_size.w)
 end
 
 function ReaderZooming:setZoom()