highlights: be less aggressive.

works fine for my single test shot. more testing would be great (+opencl version pending)
Merge branch 'master' into highlights
highlights: are still broken.
highlights: this doesn't seem half bad. needs testing.
Merge branch 'highlights'

at least this is better than what master currently has (which just
destroys your highlights and a little more if you switch on reconstruction).

i declare it as a bugfix that doesn't need to preserve the broken behaviour
for old images (tell me if you need it back for some reason)