Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

Improve OC_Image code to not guess the type of input, but actually request the specific methods to be called

Followup to #7836

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

Signed-off-by: Joas Schilling <coding@schilljs.com>

* the setup check is still there

Fix max preview, some resizing and caching issues and force preview providers to resize their previews properly
* introduces a method in OC_Image which doesn't stretch images when trying to make them fit in a box
* adds the method to all key providers so that they can do their job, as expected by the Preview class
* improves the caching mechanism of Preview in order to reduce I/O and to avoid filling the available disk space
* fixes some long standing issues
* **contains mostly tests**

Fix #7148 - again :)

Use file_get_contents() and base64_encode(file_get_contents()),
to load from data and base64, to cover more cases

data() somehow does not return, what file_get_contents() returns,
so I skipped related tests.