Attempts to move the Active Position upwards to a lesser numbered row,
or leftwards within the same row to a lesser numbered column, are now
highlighted red within the triplet list so the user can see which
triplets are being ignored due to these errors.
Detection for other triplet errors such as reserved bits/values or
object related errors will be added later on.
This is a part of moving the Active Position logic from the renderer
to the triplet list, so a renderer won't be needed if the ability to
editing (G)POP pages is added in the future.
