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Pre Medusa sprites are held in the RMA. Wimp 5.26 adjusted the RMA behaviour
where previously after *IconSprites a resize of -256MB would surprise apps
that wanted some RMA for things in their !Boot or !Run files but found there
was none due to having looked at their sprites.

However, the resize of -256MB does not always result in 0kB free space, so
each *IconSprites command ended up net growing the RMA by whatever the shrink
left *plus* the previous free amount.

Fix this by reading the free space in the RMA and adjusting up or down as
needed to always end up with a safe free space position.