save two blitbuffer memory allocations for crengine on each page turn

One for drawbuffer at CreDocument:drawCurrentView and another for
resizing of drawBuf at cre.drawCurrentPage.

base

@@ -1 +1 @@
-Subproject commit 2d733e38de74b54dd6ace933a94cbc09b40fdd4d
+Subproject commit 2a2c0f3c59f09d4deaeea554bb29ccb21a586da5

frontend/document/credocument.lua

@@ -212,10 +212,15 @@ function CreDocument:getScreenBoxesFromPositions(pos0, pos1)
 end
 
 function CreDocument:drawCurrentView(target, x, y, rect, pos)
-    tile_bb = Blitbuffer.new(rect.w, rect.h)
-    self._document:drawCurrentPage(tile_bb)
-    target:blitFrom(tile_bb, x, y, 0, 0, rect.w, rect.h)
-    tile_bb:free()
+    if self.buffer and (self.buffer.w ~= rect.w or self.buffer.h ~= rect.h) then
+        self.buffer:free()
+        self.buffer = nil
+    end
+    if not self.buffer then
+        self.buffer = Blitbuffer.new(rect.w, rect.h)
+    end
+    self._document:drawCurrentPage(self.buffer)
+    target:blitFrom(self.buffer, x, y, 0, 0, rect.w, rect.h)
 end
 
 function CreDocument:drawCurrentViewByPos(target, x, y, rect, pos)

frontend/document/document.lua

@@ -237,7 +237,7 @@ function Document:renderPage(pageno, rect, zoom, rotation, gamma, render_mode)
 
     -- prepare cache item with contained blitbuffer
     local tile = TileCacheItem:new{
-        size = size.w * size.h / 2 + 64, -- estimation
+        size = size.w * size.h + 64, -- estimation
         excerpt = size,
         pageno = pageno,
         bb = Blitbuffer.new(size.w, size.h)