fix: preserve PNG transparency in chat wallpapers

[pratikpatel8982]

Respect the user's chosen image as-is without forcing opacity, which naturally enables transparent chat background support.

fixes #29773

[CLAassistant]

All committers have signed the CLA.

[ilya-fedin]

Did you actually test it? The server accepts only jpegs so it will lose the transparency anyway, all you likely did is that the transparency will turn black instead of white.

[pratikpatel8982]

Did you actually test it? The server accepts only jpegs so it will lose the transparency anyway, all you likely did is that the transparency will turn black instead of white.

yes i tested it, the chat backround wallpaper is chosen through the chat settings,
and by setting forceOpaque=false the pipeline preserve transparency of the chat background.

1. The option Choose from File has its own file-chooser method inside settings_chat.cpp and this option was tested by me.
2. However Choose from Gallery option further expands to gallery and another Choose from file option which is taken from background_box.cpp. This was added later and is untested. However it should work the same cause they both call the same method and later go through the same pipeline.

Here is the image, a fully transparent png which was used for testing.

Here is the result: half screenshot of chat window.

here is the compiled app (linux)+ the transparent png used: https://gofile.io/d/30Ludp
It also supports semi-transparent images.