Add support for crosshairs

[ammgws]

Resolves #45

Draws fullscreen crosshairs on the output with the cursor until a selection is started.

[ammgws]

I made it so that the crosshairs are only displayed on the output with the cursor:

Or should it be displayed on all monitors that the crosshairs would intersect with?

[ammgws]

@emersion Do you have any preference regarding the above?

[y0ast]

I think the first version you suggested (only on current output) makes the most sense!

[ammgws]

@emersion anything wrong with this approach?

[emersion]

Sorry, I'll try to get some time to review this soon.

[ammgws]

@emersion ping 💐

[ammgws]

I've been using this for the past year and a bit and haven't had any issues so far.

Though one thing I haven't been able to fix is that the crosshair lines can disappear over white backgrounds.

It seems like it should be a trivial fix so perhaps someone can chime in?

[tmccombs]

Would you mind fixing the merge conflicts?

[ammgws]

@tmccombs To be honest I probably won't do it anytime soon unless it's going to get reviewed/merged since it's still working for me and I'm quite busy. I can give you write access to the fork if you want to do it.

[ammgws]

@tmccombs I didn't realise you had write access already.

Closing in favor of #95