send images as sticker not only when they have the 140x140 dimensions, but also when they have transparent corner(s)

[Simon-Laux]

also add the send as sticker logic to darg and droping an image into a chat, this makes it super convinient to create stickers from images with iOS's AI selection feature.
(long press to select your favorite pet or person then hold the object and use a second finger to navigate to the chat you want to send it to.)

[Simon-Laux]

• dropping the image to send as sticker is still unreliable
• and for some images there is still a black background (core or sth else seems to decide to convert some HEIF pictures to jpg with black background instead of transparency and some to correctly to png; maybe it has sth to do with image dimensions? I tested 2 images, the bigger one was correctly converted to png, the smaller one was converted to jpg and lost transparency).

so not ready to be merged yet.

[Simon-Laux]

I tested 2 images, the bigger one was correctly converted to png, the smaller one was converted to jpg and lost transparency

meanwhile I tested more images (create a sticker with iOS AI selection), and I haven't found another image yet where the black background appeared. It she not seem to be realtime to image dimensions, because I tried smaller images and they worked just fine.

Furthermore the transparency is preserved when I drop the image in question (with the black bg) into other apple apps, like Notes.
I suspect there is some conversion going on either in core or in deltachat-ios code before giving core the image, I haven't looked at the whole code path/flow yet (how do images get to core and treated there).

The files app says it's HEIF files, but when I send them to deltachat desktop the good ones arrive as png, while the bad one is a jpg. So maybe this HEIF issue in desktop is related?: deltachat/deltachat-desktop#3879