Window casing and conversion from inch thick drywall to half inch
Your options are:


Use a second layer of 1/2" drywall to bring the wall flush to the
window; 
Trim the window casing down by 1/2"; or 
Use inch thick
trim on the window casing.


Personally I recommend #2 and would use what's called a Japanese saw:



or a multi-tool with a wood blade



to trim the casing down to be flush with the wall.