[fix] Addon update deadlock due to race condition resulting from a CThread (CBusyWaiter) being on the stack getting destroyed prior to the actual thread exiting.
Make up missed ShowPicture in command array
AE: fix S24NE3
Improve portability
[mmal] Add MMAL decoder and MMAL renderer for Raspberry Pi

While omxplayer is the best performing player on the Pi (and the only option for raw Blu-Rays)
there are some reasons to use dvdplayer on a Pi:

DVDs with menus do not work well with omxplayer
support of software decode for SD codecs like divx3, msmpeg, sorenson spark etc
omxplayer doesn't support alsa, so can't be used with USB and I2S sound cards
dvdplayer may behave better with dodgy timestamps and packet loss, e.g. with live TV

So this PR adds hardware acceleration to dvdplayer using the MMAL api.

There is also a MMAL based video renderer. Compared to using GL, this:
saves the processing cost of YUV->RGB conversion
saves the memory cost of the RGB textures for video frames (1080p = 3MB for YUV and 8MB for RGB)
allows high quality (cubic) scaling
Merge pull request #5395 from jimfcarroll/addon-update-deadlock

[fix] Closes #15423 Addon update deadlock due to race condition deleting CThread
[readme] update master github README

This updates the master github readme to github markdown
aside from formatting added basic links and info.
Merge pull request #5426 from uNiversaI/fix-readme-typos

[readme] update master github readme
Merge pull request #5354 from popcornmix/mmal_acceleration

[mmal] Add MMAL decoder and MMAL renderer for Raspberry Pi
Merge pull request #5404 from ulion/show_picture_cmd
Merge pull request #5446 from FernetMenta/s24

AE: fix S24NE3
Merge pull request #5437 from veger/posix-fix

Improve portability
[posix] [vfs] Implement PosixFile class for accessing local files on Posix platforms
[posix] [vfs] Switch to new PosixFile
[vfs] Remove HDFile
Merge pull request #5309 from Karlson2k/vfs_posixfile_01

[posix] Implement new PosixFile
[omx] Fix compile warning.

This fixes commit 77778e4e1ef549455574f765ff6252fbb278effa.
Merge pull request #5452 from anaconda/fix-pr5348

[omx] Fix compile warning after PR 5348.