linux/x11: Set transparency to false by default (#13848)

The previous code lead to a ton of error messages from Blade on my X11
machine, even with *client-side decorations working well!*

As @someone13574 pointed out
[here](https://github.com/zed-industries/zed/pull/13611#issuecomment-2201685030)
things still work with this being false. And if someone changes a theme
with transparent background, then it will get set anyway. We just don't
do it by default.

And as @jansol pointed out
[here](https://github.com/zed-industries/zed/issues/5040#issuecomment-2096560629):

> On X11 it may be possible to configure a compositor to blur window
backgrounds but Zed has no way to influence that.

So we don't lose anything, I think, but get rid of a ton of error
messages in the logs.

Proof of shadows etc. still working:

![Screenshot from 2024-07-05
10-17-38](https://github.com/zed-industries/zed/assets/1185253/1216b38b-8011-46e7-b86f-c0f5fc3f6f64)



Release Notes:

- N/A