initial implementation of slideshow view
slideshow: coarse view skeleton
slideshow: create background thread to process image buffer.
slideshow: fix crash.
slideshow: comments about the plan
slideshow: get screen dimensions and alloc double buffer
slideshow: fix mem allocation
slideshow: process images, click left and right
slideshow: use display profile
slideshow: add some locks around thread shared data.

also center image.
slideshow: remove debug output
slideshow: hide borders around image
slideshow: fix centering issue
slideshow: stupid synchronization for backbuf processing.

also fix alignment problem of dt_log

also broke right mouse click (go back) again, since next image is now
already processed.
slideshow: todo about async picture advancing
slideshow: introduce state machine to sync bg job and ui
slideshow: backwards (right click) works.
slideshow: add option for hq processing
slideshow: start at current lighttable image
slideshow: global key accel to enter slideshow : `s'