Fixing graphical and stability issues with GTK 4 Flatpak applications

This problem is caused by a bug on Mesa driver version 23.3 and makes apps unusable. There's two ways of workaround the issue:

• downgrading Mesa to version 23.1;
• setting the enviroment variable FLATPAK_GL_DRIVERS=host for affected apps.

The last one may cause problems in some apps, like Telegram, which broke its OpenGL media acceleration. To prevent that, do not set the enviroment variable globally.