Increase im->watermark buffer size, so it can fit some pango markup

I think that its a good idea to also increase other parts of the im structure and threat their size with sizeof, instead of current fixed numbers.

If you agree to accept such a change, let me know, I will prepare patch for that too.
I have no idea too - I was going to ask you the same question...

The logic to have 10 spare bytes at the end of the buffer was present before. I just increased the original buffer from 110 to 1010 and preserved the logic to have 10 spare bytes. 

If its not needed, then why waste memory?