gradientsliders: use NAN as value to disable display of color picker value

this allows better handling of out-of-bound values - those are clamped to
the gradient slider range but still a marker is displayed in the slider
colorin: add option to normalize Lab values to real, ie. non-virtual colors

depending on the color matrix belonging to a camera our current code may deliver
"virtual" Lab colors, e.g. pixels with negative L and high a/b values. while this
liberal handling of out-of-bound colors most frequently delivers good results, in
some cases our modules may produce serious artifacts. typically you find this in
photos showing saturated blue light sources (tell: stage photography) and when using
certain modules like "levels". the new code offers an option to normalize colors,
i.e. restrict Lab to really existing colors. we achieve this by transforming
camera RGB to linear RGB, clamping, and finally transforming to Lab. currently
if the normalize option is activated all colorin processing is done by lcms2
which leads to a significant slow-down.
colorin: extended version: fix shortcomings; enable selection of clipping profiles; implement SSE and OpenCL codepaths
colorin: some code cleanup
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