fix multiple bugs in scrot's -C option

[guijan]

I've carefully read the Xlib manuals and some more, but this PR should be reviewed by someone more familiar with X programming.

[N-R-K]

Unfortunately I cannot test this since the -k options seems to require a compositor (and I don't want to go though the hassle of setting one up).

Can you confirm that you did some before/after testing (just in case) @guijan ? If yes, then I'll go ahead and merge this since the code looks OK to me.

[guijan]

Yeah, I've tested it. Here's a screenshot I just took.

[N-R-K]

Thanks.