Do not modify a const GdkEvent in place

Do not modify a const GdkEvent in place

When deciding whether or not to emulate a press event, we're translating
the last event coordinates and mutating the given event structure
unconditionally.

We should modify the newly created GdkEvent copy, since it's what we're
going to use when emitting the press event.

This avoids mutating a constant GdkEvent and global state, and also
avoids a compiler warning.