A few minor fixes after #4847 (#4850)

* Add a toggle to disable the C blitter in the Dev menu (depends on https://github.com/koreader/koreader-base/pull/882) (never shown if the JIT is disabled, grayed out if the C blitter is not installed)
* Fix a few sizeUtf8Text call sites that were doing a nil check in order to account for the new return type.
* Tweak statusbar handling to avoid spurious sizeUtf8Text warnings when it's hidden, and unify its behavior between being hidden via toggle, and hidden on book open (at least when all-at-once is not enabled).
* c.f., https://github.com/koreader/koreader-base/pull/882 (Android, PB, RGB32 & Legacy Kindle regression fixes).

frontend/apps/filemanager/filemanagermenu.lua

@@ -260,6 +260,29 @@ function FileManagerMenu:setUpdateItemTable()
             end,
             callback = function()
                 G_reader_settings:flipNilOrFalse("dev_startup_no_fbdepth")
+                local InfoMessage = require("ui/widget/infomessage")
+                UIManager:show(InfoMessage:new{
+                    text = _("This will take effect on next restart."),
+                })
+            end,
+        })
+    end
+    if not Device.should_restrict_JIT then
+        table.insert(self.menu_items.developer_options.sub_item_table, {
+            text = _("Disable C blitter"),
+            enabled_func = function()
+                local lfs = require("libs/libkoreader-lfs")
+                return lfs.attributes("libs/libblitbuffer.so", "mode") == "file"
+            end,
+            checked_func = function()
+                return G_reader_settings:isTrue("dev_no_c_blitter")
+            end,
+            callback = function()
+                G_reader_settings:flipNilOrFalse("dev_no_c_blitter")
+                local InfoMessage = require("ui/widget/infomessage")
+                UIManager:show(InfoMessage:new{
+                    text = _("This will take effect on next restart."),
+                })
             end,
         })
     end