The 45 degree cut is for boards that meet each other at right angles - i.e., 90 degrees. If your boards meet at 135 degrees they would need LESS of a mitre, probably 22.5 degrees. (Boards meeting at 180 degrees need no mitre - just a plain right angle cut)

In other words, I think the saw can handle your angles no problem. If you set it at 22.5 for both cuts and hold them together, the resulting angle should be 135 degrees.

135 degrees is the same as a 45 degree. A straight wall is 180 degree a standard corner is 90 degree. As Tryinbrian said, cut 22.5 degrees and you have your 45. 180 less 135 is 45 degrees. You can cut any angle on the saw you have.