Code review comment for lp:~fboucault/thumbnailer/new_qml_api

Let me try to condense Florian's concerns for those who have not had to deal with the process of thumbnailing.

Creating a video screenshot thumbnail takes several seconds (up to 5) on the device.

Creating a thumbnail for other types (mostly images) takes a fraction of a second. Loading a preexisting thumbnail takes roughly the same.

Suppose you have n threads for thumbnailing and want to display n videos and m images. If you submit the videos first, the images are stuck until the first video gets processed.

What we want is to be able to submit both images and videos and have the system prioritise those thumbnails that are quick to load (like, say, reserving one thread for them and one for video screenshots).

Qt's image provider does not have priorisation capabilities and to fix our use case something like that needs to be added to Qt. This is the Correct thing to do, but on the other hand it is a lot more work than just increasing the amount of threads for QQuickImageProviders.