Recolour sprites are loaded when seen, instead of being loaded when needed. This could result in the sprite cache being filled up with recolour sprites, and also mean that replacing recolour sprites didn't release the previously allocated memory.
Instead, allow recolour sprites to be loaded as needed and freed when unneeded, like regular sprites.
This fills up the sprite cache with SPR_OPENTTD_BASE + OPENTTD_SPRITE_COUNT zero-size sprites, to
allow GetSpriteSize() calls to continue from unit-tests.
Peter Nelson