Fix alt screen bugsissue-2145

This fixes two bugs with the alternate screen buffer.

When resetting while in the alt screen, Alacritty would not swap out
the grids leading to scrollback getting disabled. By swapping out the
grids again when resetting in the alternate screen buffer, scrollback is
now unaffected from a reset.

There was another issue with the cursor jumping around when leaving the
alt screen even though it was not active, this was fixed by skipping all
alt screen swap routines unless the current state matches the expected
state.

This fixes #2145.

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