wonder if there is a dremel tool for cutting glass

Such a cutting system would be way overmatched. Instead, buy a regular glass
cutting (scoring) tool. Using the duct tape technique for safety, start at one
side with a scored line, tap it with the tool as glass pros do (to propagate
the crack) and gently pry the section away from the wall. Repeat as necessary
with some common sense precautions like having a robust container handy,
covering the floor with cardboard to keep glass shards from damaging it, etc.
HTH

Joe