lighttable: enable full-res thumbnail when pressing `z'.

unfinished, needs better user interface/option in prefs for it.
preview: fix memleak and some rounding errors
preview: fall back to old `z' in case there is no thumbnail in the file.
Merge remote-tracking branch 'origin/master' into full-res-preview
preview: don't jump when mouse exits drawing area.
preview: take orientation into account.
preview: initial sharpness detection code.
focus: initial focus cluster regions implementation.

needs weighted line width/color to indicate sharpness
focus: some attempt to rate and visualise sharpness regions
focus: try two-level cdf22 wavelett.

so far a lot worse than the simple version.
focus: some tweaks to sharpness detection.
Merge remote-tracking branch 'origin/master' into full-res-preview

Conflicts:
	src/views/lighttable.c
focus: make clusters detect more robustly, and draw over regular thumb.

red: spot on focus
blue: one wavelet level later (for high iso shots)
focus: adapt focus regions to reflect distortions/transforms in the pipe.
Merge remote-tracking branch 'origin/master' into full-res-preview
focus: replace wrong boxes with rounded quads.

better reflect the axes and how they get distorted through the pipe. not sure about the rounded corners yet.
Merge remote-tracking branch 'origin/master' into full-res-preview
focus: some visualization changes. not final yet.
focus: slight code cleanup.

factored out focus relevant stuff into their own header.
focus: deleted unused code.