renderer: reset render buffers on flush
winrenderer: implement flush, fixes loss of video when moving window to other screen
vaapi: lock display when calling vaPutSurface
vaapi: call glFinish before closing context
fix setting resolution on startup
Merge pull request #5427 from FernetMenta/fullscreen2

fix setting resolution on startup
vaapi: correct dimensions of the various buffers
Merge pull request #5420 from FernetMenta/winrender

Winrender: fix moving window between screens
Merge pull request #5424 from FernetMenta/vaapifix

vaapi: stability
fixed: Uninitialised variable in VAAPI
Merge pull request #5433 from arnova/vaapi_fix

fixed: Uninitialised variable in VAAPI
VAAPI: fix some cppcheck oddities
Merge pull request #5435 from fritsch/vaapi-cppcheck

VAAPI: fix some cppcheck oddities
videoplayer: adapt lateness detection and dropping to buffering
video player: present correct pts to user for a/v sync (after buffering in renderer)
videoplayer: update frametime, it might change due to fps detection
videoplayer: give streams with invalid fps a chance for fps detection
dvdplayer: allow rewinding at end of stream, do a seek after rewind
dvdplayer: observe pts counter overflow
dvdplayer: avoid short screen flicker caused by unnecessary reconfigure of renderer