The crate containing the entry point is called alacritty, and the crate
containing everything else is called alacritty_terminal.

Previously Alacritty has initialized all lines in the buffer as soon as
it is started. This had the effect that terminals which aren't making
use of the scrollback buffer yet, would still consume large amounts of
memory, potentially even freezing the system at startup.

To resolve this problem, the grid is now dynamically resized in chunks
of `1000` rows. The initial size is just the visible area itself, then
every time lines are written to the terminal emulator, the grid storage
is grown when required.

With the worst-case scenario of having 100_000 lines scrollback
configured, this change improves startup performance at the cost of
scrolling performance.

On my machine the startup changes from ~0.3 to ~0.2 seconds.

The scrolling performance with large throughput is not affected, however
it is slowed down when the number of lines scrolled are close to the
100_000 configured as scrollback. The most taxing benchmark I've found
for this was running `yes | dd count=500 > 500.txt` (note the relatively
small file size). This will cause a slowdown on the first run from 0.05s
to 0.15s. While this is significant, it lines up with the time saved at
startup.

This fixes #1236.

Now, only cells that have been used are cleared. This is achieved by
using a "occupied" memo on the Row itself. The value, `occ`, is updated
wherever the Row is accessed mutably, and it's cleared to zero in
Row::reset.

The tests for grid scroll_up and scroll_down were updated to include a
test on the value `occ` and slightly refactored, but are otherwise
equivalent to the previous implementation of those tests.

Because of the change to the `Row` struct, the ref tests were updated so
Deserialization keeps working as expected.

The old grid_reset test expected the complete grid to be reset, but
instead of resetting the whole grid the `grid.scroll_limit` is just set
to `0` now. This leads to the rest of the grid still containing the old
information.

To fix this test it has simply be re-recorded. The new tests now still
contains the complete history but it is checked that the `scroll_limit`
is correctly reset.

In the current scrollback PR the `reset` command does not affect the
scrollback history. To make sure the terminal is properly reset, it
should clear the scrollback history.

This commit fixes this by creating a new and empty grid whenever `reset`
is executed. It takes the current dimensions and history size from the
old grid.

Right now there's an empty ref-test called `grid_reset` without any
content, this should be implemented once #1244 is resolved.

This fixes #1242.