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).

reader.lua

@@ -34,6 +34,24 @@ if lang_locale then
     _.changeLang(lang_locale)
 end
 
+-- Make the C blitter optional (ffi/blitbuffer.lua will check that env var)
+local ffi = require("ffi")
+local dummy = require("ffi/posix_h")
+local C = ffi.C
+if G_reader_settings:isTrue("dev_no_c_blitter") then
+    if ffi.os == "Windows" then
+        C._putenv("KO_NO_CBB=true")
+    else
+        C.setenv("KO_NO_CBB", "true", 1)
+    end
+else
+    if ffi.os == "Windows" then
+        C._putenv("KO_NO_CBB=false")
+    else
+        C.unsetenv("KO_NO_CBB")
+    end
+end
+
 local Device = require("device")
 local dpi_override = G_reader_settings:readSetting("screen_dpi")
 if dpi_override ~= nil then