Cutting Mirror Already Glued to Wood (Fiber Board)

Ok, I have the VIKEDAL doors from IKEA, the long one, measurements

• Width: 19 5/8"
• Height: 90 1/8"

and I want to cut it to a shorter length. I have plenty of tools to cut glass easily but idk how I would cut it if it's already glued to wood (that compact wood chips thing, I think its called fiberboard).