Faster blitting @ BB8/BBRGB32 when no processing is needed (#4847)

* Pickup the eponymous blitting performance tweaks from koreader/koreader-base#878
* Cleanup BitOpts usage (require & cache)
* Unify oddness checks (MOD -> AND)
* Enforce the native Portrait orientation on Kobo (except @ 16bpp, i.e., KSM w/ 8bpp swap disabled), to allow for faster blitting when unrotted.
* Switch CRe BB to 32BPP on color screens
* Minor cleanups

frontend/document/document.lua

@@ -134,6 +134,7 @@ function Document:fastDigest(docsettings)
         local result = docsettings:readSetting("partial_md5_checksum")
         if not result then
             logger.dbg("computing and storing partial_md5_checksum")
+            local bit = require("bit")
             local md5 = require("ffi/MD5")
             local lshift = bit.lshift
             local step, size = 1024, 1024