Layout bugs in attachments

[llsc12]

Attachments can cause layout cycles due to their non-finite size. The non-finite size is a result of unloading when not on screen (probably?), causing MarkdownText to recompute layouts in a loop and causing what appears to be a hang to end-users.

This should be fixed by using geometryreader in conjunction with the attachment's size data to make a deterministic frame.

This will be fixed once attachment viewer work is in place and attachments in messages are reworked.

[llsc12]

This also happens with embeds.

[llsc12]

perhaps the issue was to do with markdowntext all along?

[llsc12]

#10 might address broken embed layouts and stuff, however embeds do also need fixing as they do a lot of complicated layouts.

[llsc12]

after optimising markdowntext, a lot of issues have disappeared. plans are still in place to improve embeds views

[llsc12]

Rundown of the situation:
MarkdownText gets initialised a lot. Attachments and Embeds can sometimes throw MarkdownText into a layout cycle because of SDWebImageSwiftUI's AnimatedImage sizing behaviours. The images also tend to lag a lot when scrolling in. This affects member lists too. Also on a separate note, the scrollview used in the chat is really bad and needs replacing with platform native scrollview.