monitor-config-store: Discard config with fractional scale when unusable

When a configuration has a fractional scale, but we're using a physical monitor layout, we can't use the scale, but if we do, we end up with wierd issues down the line. Just discard the config if we run into this.

Eventually we probably want to store the layout mode in the configuration so we can handle more seamless switching between physical and logical layout mode, but first do this.

Closes: #3057 (closed)

Draft as I haven't tested it.

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