send ad-hoc stickers as stickers

[r10s]

iOS allows to create a "sticker" from each image by a long tap. with a second finger, you can then navigate to Delta Chat and send that. unfortunately as non-transparent image and not as sticker:

RPReplay_Final1722598584.MOV

these stickers were used to be sent as stickers, however, because of #2128 we decided to stage images if in doubt - which is still the better approach.

still, it we can improve the detection of a "sticker", which is currently only by size. instead/additionally, we can check if eg. corner pixels are transparent and assume a sticker in that case.

this was already suggested, however, postponed until we see it is really needed, which is the case now

EDIT: that does not seem to be directly related, see below

[r10s]

hm, seems not to be related to the detection of #2128 , that code path seems not to be executed when dropping an image that way. still, would be interesting which code path is involved there and if we can tweak things easily there.

the detection of #2128 is working as expected when having added the image as a sticker before:

RPReplay_Final1722600093.mov