Make sure getting rid of a DPI override actually restores default behavior (#6984)

* Make switching *back* to the default DPI via auto actually work
properly.

eg., by actually setting DPI back to defaults, and clearing the override
flag.

* Bump base to pickup https://github.com/koreader/koreader-base/pull/1258

base

@@ -1 +1 @@
-Subproject commit 5d2695b520977a48066043e8e1712cced90e0d15
+Subproject commit ab6cf44acb60e6db7cf4c152af523272d47b0117

frontend/device/generic/device.lua

@@ -205,9 +205,9 @@ function Device:init()
 end
 
 function Device:setScreenDPI(dpi_override)
+    -- Passing a nil resets to defaults and clears the override flag
     self.screen:setDPI(dpi_override)
     self.input.gesture_detector:init()
-    self.screen_dpi_override = dpi_override
 end
 
 function Device:getPowerDevice()

frontend/ui/elements/screen_dpi_menu_table.lua

@@ -3,7 +3,7 @@ local Device = require("device")
 local Screen = Device.screen
 local T = require("ffi/util").template
 
-local function isAutoDPI() return Device.screen_dpi_override == nil end
+local function isAutoDPI() return Screen.dpi_override == nil end
 
 local function dpi() return Screen:getDPI() end
 
@@ -16,7 +16,9 @@ local function setDPI(_dpi)
         text = _dpi and T(_("DPI set to %1. This will take effect after restarting."), _dpi)
                or _("DPI set to auto. This will take effect after restarting."),
     })
+    -- If this is set to nil, reader.lua doesn't call setScreenDPI
     G_reader_settings:saveSetting("screen_dpi", _dpi)
+    -- Passing a nil properly resets to defaults/auto
     Device:setScreenDPI(_dpi)
 end