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/document/koptinterface.lua

@@ -540,10 +540,11 @@ end
 --[[
 get page regions in native page via optical method,
 --]]
-function KoptInterface:getPageRegions(doc, pageno)
+function KoptInterface:getPageBlock(doc, pageno, x, y)
+    local kctx = nil
     local bbox = doc:getPageBBox(pageno)
     local context_hash = self:getContextHash(doc, pageno, bbox)
-    local hash = "pageregions|"..context_hash
+    local hash = "pageblocks|"..context_hash
     local cached = Cache:check(hash)
     if not cached then
         local page_size = Document.getNativePageDimensions(doc, pageno)
@@ -553,17 +554,19 @@ function KoptInterface:getPageRegions(doc, pageno)
             y1 = page_size.h,
         }
         local kc = self:createContext(doc, pageno, bbox)
-        kc:setZoom(1.0)
+        local screen_size = Screen:getSize()
+        -- leptonica needs a source image of at least 300dpi
+        kc:setZoom(screen_size.w / page_size.w * 300 / self.screen_dpi)
         local page = doc._document:openPage(pageno)
         page:getPagePix(kc)
-        local regions = kc:getPageRegions()
-        Cache:insert(hash, CacheItem:new{ pageregions = regions })
+        kc:findPageBlocks()
+        Cache:insert(hash, CacheItem:new{ kctx = kc })
         page:close()
-        kc:free()
-        return regions
+        kctx = kc
     else
-        return cached.pageregions
+        kctx = cached.kctx
     end
+    return kctx:getPageBlock(x, y)
 end
 
 --[[