fix Tusky sometimes displaying empty tabs (#5007)

Sometimes a tab is just empty after switching to it, but content appears
as soon as you interact with it.
I found that setting a `offscreenPageLimit` on the `ViewPager2` makes
that issue go away.
This also has the side effect that the content of more tabs is
preloaded, which makes tab switching *feel* different, one does not see
so many loading spinners, content is just there instantly. I'm not sure
if that is desireable but I would like to try it out for a while. If it
happens to be a problem we can still move tab initialization to when it
is actually visible.

app/src/main/java/com/keylesspalace/tusky/MainActivity.kt

@@ -342,6 +342,7 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, MenuProvider {
         // https://github.com/tuskyapp/Tusky/issues/3251 for details.
         tabAdapter = MainPagerAdapter(emptyList(), this@MainActivity)
         binding.viewPager.adapter = tabAdapter
+        binding.viewPager.offscreenPageLimit = 2
 
         lifecycleScope.launch {
             viewModel.tabs.collect(::setupTabs)