BMP support

[anlutro]

I'm effectively running these commands:

scrot /tmp/lock.bmp
convert /tmp/lock.bmp -scale 10% -scale 1000% /tmp/lock.png
i3lock -i /tmp/lock.png

This is somewhat slow, though:

$ time scrot /tmp/lock.bmp && time convert /tmp/lock.bmp -scale 10% -scale 1000% /tmp/lock.png

real    0m0.183s
user    0m0.128s
sys 0m0.024s

real    0m0.357s
user    0m0.328s
sys 0m0.024s

If I ditch the convert to PNG step it's a lot faster:

$ time scrot /tmp/lock.bmp && time convert /tmp/lock.bmp -scale 10% -scale 1000% /tmp/lock.bmp

real    0m0.178s
user    0m0.128s
sys 0m0.020s

real    0m0.163s
user    0m0.124s
sys 0m0.036s

However, i3lock doesn't support BMP.

[Airblader]

Please see #68 and #157. The basic argument is the same for BMP.