Fix video in slideshow not show and hang in black problem.
[Picture] add methods to CSlideShowPic for reuse a loaded picture.
[SlideShow] use GetNextSlide() to get next slide number as long as possible.
Slideshow show video thumb when pausing.
Allow screen saver when slideshow pausing.
Consider 'unplayable' property when GetNextSlide().
Try to skip to next when playing slideshow and current slide is 'unplayable'.
Slideshow check picture path on picture loaded.
Add method to get slideshow number and pic slot for the background pic loader.
Only update texture when was loaded for reload.
No need to close next loaded image when zoom/reload current one.
Only reload current image when not switching to next and current is not video.
Move display effect code into standalone method GetDisplayEffect.
Always close next loaded pic if not matching slidenumber.
Rewrite code for switching to next slide.
Slideshow rewrite the error handling code.
Change CGUIWindowSlideShow::Select() to use same condition with code
loading first picture in Process().
Change AnnouncePlayerPlay to announce speed == 0 for pausing slideshow or picture.
Keep slide infos during video playback.
Unify picture display effect during slideshow or pausing, browsing.