fix(tui): reset clear_region attributes during startup

[luukvbaal]

Problem: Fix added in #28676 worked accidentally(used variables were
themselves uninitialized at this point during startup) and
does not always work.
Solution: Reset attributes when clearing regions during startup.

[gpanders]

@luukvbaal IIUC the current behavior on master is fine as-is and this PR fixes an issue when a redraw is performed in _defaults.lua? So kicking this to the 0.11 milestone should be ok?

[luukvbaal]

Flush, not redraw. But yeah for reference: I noticed the test repro still producing a black screen on my ext_ui branch, which calls vim.ui_attach()->ui_flush() at the top of _defaults.lua. Adding any flush before the tty shenannigans in _defaults.lua may still produce a black screen(don't ask me why).

That isn't to say that I know for sure that that is the only way to still produce a black screen during start up. So in that sense this fix seems safer but if no one dares press merge before 0.10, after works too.

[gpanders]

(don't ask me why)

This is really the root of my hesitation to merge this pre 0.10. If it doesn't fix any user facing bugs (that we know of) in master then it should be safe to slow down and find a root cause. I'd like to understand this too, but I don't have the bandwidth to chase it down in the next day or two, so my preference is to kick the can to the 0.11 release cycle.

[luukvbaal]

Yeah fair. I kind of lost interest in debugging the behavior on master further once I found this alternative solution and established that #28676 made incorrect assumptions.

Figuring out why trying to force clear_region()->update_attrs() to use the default background by initializing to 0/-1(which it turned #28676 actually amounted to), may still be interesting but I'm not sure how useful that actually still is give it's accidental nature. The updated test output(actual blank screen vs some attr) also seems more reliable.