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.
ini-key must be present if WWT_DEFSIZEBOX or WWT_STICKYBOX is present.
This was previously enforced by a workflow, however that parsed the source
code with regex which turned out to be error-prone.
