Page Overlap: Fix rectangle computation and arrow mode (#7269)

* In ReaderPaging, the panning step pre-PanningUpdate can be wildly overshot near page edges, so, use the corrected value instead by recomputing it after the panning has been effectively computed by ReaderView.
This fixes slight inaccuracies, as well as glaring mistakes when going backwards, or when near page edges.
This is in line with how ReaderRolling computes the value, which I only realized later because I'm an idiot. 
* Minor cleanups around the handling of the dim_area Geom object in general.

* Fix the "Arrow" page overlap mode to be painted in the right coordinates when going backward. Issue might not have been terribly clear because of the previous issue ;).
* Center the arrow's point, while we're here.
* Don't use AlphaContainer to make it translucent, because AlphaContainer is horribly broken, and has weird quirks and behavior that make no sense to me unless some very specific and unlikely constraints are met, and they definitely aren't here.
This fixes the arrow copying an arrow-sized square of the original page the book was opened on on the top-left corner of *every* page with an arrow. (lol).
* Do real proper alpha-blending via Icon/ImageWidget from the original icon, instead of faking it via addBlitFrom, in order to avoid the dimming *around* the triangle's shape.

frontend/apps/reader/modules/readerview.lua

@@ -2,7 +2,6 @@
 ReaderView module handles all the screen painting for document browsing.
 ]]
 
-local AlphaContainer = require("ui/widget/container/alphacontainer")
 local Blitbuffer = require("ffi/blitbuffer")
 local ConfirmBox = require("ui/widget/confirmbox")
 local Device = require("device")
@@ -80,17 +79,13 @@ function ReaderView:init()
     -- fix recalculate from close document pageno
     self.state.page = nil
     -- fix inherited dim_area for following opened documents
-    self:resetDimArea()
+    self.dim_area = Geom:new{w = 0, h = 0}
     self:addWidgets()
     self.emitHintPageEvent = function()
         self.ui:handleEvent(Event:new("HintPage", self.hinting))
     end
 end
 
-function ReaderView:resetDimArea()
-    self.dim_area = Geom:new{w = 0, h = 0}
-end
-
 function ReaderView:addWidgets()
     self.dogear = ReaderDogear:new{
         view = self,
@@ -105,14 +100,13 @@ function ReaderView:addWidgets()
         ui = self.ui,
     }
     local arrow_size = Screen:scaleBySize(16)
-    self.arrow = AlphaContainer:new{
-        alpha = 0.6,
-        IconWidget:new{
-            icon = "control.expand",
-            width = arrow_size,
-            height = arrow_size,
-        }
+    self.arrow = IconWidget:new{
+        icon = "control.expand.alpha",
+        width = arrow_size,
+        height = arrow_size,
+        alpha = true, -- Keep the alpha layer intact, the fill opacity is set at 75%
     }
+
     self[1] = self.dogear
     self[2] = self.footer
     self[3] = self.flipping
@@ -175,14 +169,16 @@ function ReaderView:paintTo(bb, x, y)
     end
 
     -- dim last read area
-    if self.dim_area.w ~= 0 and self.dim_area.h ~= 0 then
+    if not self.dim_area:isEmpty() then
         if self.page_overlap_style == "dim" then
             bb:dimRect(
                 self.dim_area.x, self.dim_area.y,
                 self.dim_area.w, self.dim_area.h
             )
         elseif self.page_overlap_style == "arrow" then
-            self.arrow:paintTo(bb, 0, self.dim_area.h)
+            local center_offset = bit.rshift(self.arrow.height, 1)
+            -- Paint at the proper y origin depending on wheter we paged forward (dim_area.y == 0) or backward
+            self.arrow:paintTo(bb, 0, self.dim_area.y == 0 and self.dim_area.h - center_offset or self.dim_area.y - center_offset)
         end
     end
     -- draw saved highlight
@@ -594,8 +590,7 @@ function ReaderView:recalculate()
             self.visible_area:offsetWithin(self.page_area, 0, 0)
         end
         -- clear dim area
-        self.dim_area.w = 0
-        self.dim_area.h = 0
+        self.dim_area:clear()
         self.ui:handleEvent(
             Event:new("ViewRecalculate", self.visible_area, self.page_area))
     else