Currently dx9screencapsrc prints a verbose warning in case the screen
index is out of range for the current number of detected monitors. This
value is then dropped.
However there is no initial indication (beside the console print) if it
worked or not. This may result in capturing an unwanted screen as it
would capture the last set index that was not rejected.
This patch sets the index regardless. Instead, the element throws an
error when it tries to run or getting caps for an invalid index.
https://bugzilla.gnome.org/show_bug.cgi?id=771817
1489e73df4