compil against external libbluray
[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
[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
[omxplayer] Fix bad audio when sample_fmt is not reported on Open

For most codecs ffmpeg reports m_pCodecContext->sample_fmt on the avcodec_open2 call (e.g. ac3, dts, aac).
However some (PCM on dvd) report AV_SAMPLE_FMT_NONE on open, and only provide the correct format on decode.

This can cause the GetBitsPerSample call to change between open and decode and results in white noise when the word size is misinterpreted.

The simple solution is to always convert to float when open doesn't report the format and ensure GetBitsPerSample reports this value
[settings] Rename DVDs category to Discs and add an option to specify the handling of Blu-rays.
[FileItem] Also check for MovieObject.bdmv in IsBDFile().
[bluray] Use "disc.playback" setting to start Blu-ray playback.
Treat bluray directories like images and discs when playing them via the context menu.
Merge pull request #5228 from ace20022/blu_select2

[bluray] Add a setting for preferred bd playback
[bluray] Allow retrieval of arbitrary items from a Blu-ray directory.
[ext. subs] Also check for external subtitles for Blu-ray disc image files.
Merge pull request #5204 from ace20022/bd_extsubs

[Fix] Make ext. subtitle loading work for Blu-ray disc image files and Blu-ray directories.
Merge pull request #4153 from alexmaloteaux/configure_libbluray_fix

compil against external libbluray
Merge pull request #5442 from popcornmix/omx_pcm_fix

[omxplayer] Fix bad audio when sample_fmt is not reported on Open
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