Fix Firewall/Flamewave/Lightningwall not working on every tile

[obligaron]

Fixes #6189
Fixes #6190

Notes

• When casting Firewall, Lightningwall, or Flamewave in the north, south, west, or east direction, an extra segment is added.
• For Firewall and Lightningwall, the extra segment only damages walking players and monsters.
• The extra segment is currently invisible but could be rendered. This is not part of this PR because it changes the visuals, and there are some graphic glitches (see the last example videos).
• The first commit harmonizes the growth logic between Firewall and Flamewave (no behavior changes).
• The second commit adjusts the methods called in CheckMissileCol to get references instead of indexes (no behavior changes).
• The third commit adds gaps as necessary.
• The fourth commit ensures that the newly added gaps in the firewall and LightningWall only damage walking monsters and players.

Example Videos

Firewall

Master

Master_Firewall.mp4

PR

PR_Firewall.mp4

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Flamewave

Master

Master_Flamewave.mp4

PR

PR_Flamewave.mp4

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Lightningwall

Master

Master_Lightningwall.mp4

PR

PR_Lightningwall.mp4

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With rendered gaps

Firewall / Flamewave

PR_WithVisibleGap.mp4

Lightningwall

PR_WithVisibleGapLightning.mp4

[AJenbo]

Nice to have a solution for this :)