Add copy draw

[jcupitt]

resync with nip4, again

[jcupitt]

@kleisauke I think this (no, really) might fix the info bar crash. I've not been able to make it fall over myself yet anyway.

You'll need the matching add-copy-draw libvips branch to test.

• uses _copy_draw() and not _inplace(), so it should be more threadsafe
• paint actions are mostly in-line now and don't trigger invalidation (you have to do this manually)
• centralised progress reporting
• much faster painting

[kleisauke]

Great! I tried my best to make it crash, but it didn't, which is a good thing! :)

I only noticed one minor issue: the progress bar doesn't show when opening st-francis.jpg in the GUI, while vipsdisp st-francis.jpg works as expected. I suspect there's a missing gtk_action_bar_set_revealed() call somewhere.

I'll review the corresponding libvips PR later, but I noticed some CIFuzz failures there (not sure if you'd seen those).

[jcupitt]

Great! Thank you for testing!

Yes, I noticed the missing progress bar for "replace" too. I've not found a fix yet :(

I think it's something like event starvation: progress.c emits begin/update/end from an idle handler, and idle handlers won't run if the main thread is too busy.

[jcupitt]

Oh wait, "replace" isn't using the same load path, that's why it's starving events. I'll try to fix it.