Merge pull request #554 from MRIG/master

Updated Dutch translation
Fix #9948: Don't optimize isnan() away

I wonder why they broke this and what will be next ...
First batch of high DPI fixes

Soem small things like the triangles in our custom dr widgets are still
missing, but the big issues should be solved now.
I only tested these thigns by setting the screen DPI using xrandr, I am
not 100% sure if that is a perfect simulation.
People using a high DPI screen should still set the panel_width in the
config to something bigger.
Fix libpng-1.4 build
fix several translating errors
fix base curve mismatching
update chinese translation
Merge pull request #555 from cherrot/i18n_zh_CN

I18N: Update Chinese translations. Fix several translating errors
High DPI: More fixes, this time cairo stuff

TODO:
 - color picker
 - masks
 - anything else?
Italian translation updated
maybe nitpicky, but I think it's more correct. changed 'and' to 'or'.
added commas, and made some minor wording changes.
edited sentences (and a paragraph break) for clarity.

Some parts in here, I had to reread the text a couple times to make
sure I understood. I think it is clearer the way I've changed it.
Update French translation.
Merge pull request #556 from bartokk/master

Italian translation updated
Lens IOP: add autodetection, based on fact "did user or did not changed anything in GUI from autodetected values"
Merge pull request #548 from LebedevRI/lens-autodetection

Lens IOP: add autodetection
Some more GUI fixes

- Try to keep background grids crisp in custom widgets.
- Put a subdued dt logo into the empty widget of zone system.
Prevent crash on startup. Preliminary fix! I still need to actually reproduce original crash!
https://github.com/darktable-org/darktable/pull/548#issuecomment-42763922
Merge pull request #557 from Terrycloth/clarifying-text-in-user-manual

my first few edits to the user manual.