This is needed for the upcoming decoder write as decode.cpp will track the
Active Position from the internal data of x26triplets.
If decode.cpp is rewritten again to track the Active Position independently
of x26triplets this commit may be reverted, but then again tracking how the
Active Position moves on both levels 1.5 and 2.5+ may be useful in terms of
warning the user of differing results on different decoder levels.
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.
