Add simpler elevation tileset

[manuq]

Plus a separate tiles/decoration.tres tileset for decoration tiles
that can be added in a layer on top of other tiles.

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Duplicate tiles/elevation.tres into tiles/elevation_2.tres and
simplify it:

Set size in atlas of tiles with walls below to be 1x2 (twice taller)
and adjust their rendering texture origin to be -32 pixels.

Adjust the collision polygons of the now taller tiles to be the same
as before. Both in physics layer 0 (matching "walls") and in physics
layer 1 (matching "sight occluders"). Also remove physics layer 2
that was matching "non-walkable floor" but didn't have any polygons
in tiles.

Leave a single "Stone" terrain that can paint all at once and with
consistency for the internal wall corners.

Add a new alternative tile for the center tall tile, so it can
be used eg. in Fray's End to let the player walk the bridge.

Remove cliff mines tiles by removing the PNG from tile sources.

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For decoration tiles:

Create a new PNG using GIMP to remove the duplicated pixels between
cliffs with and without mines. This is done by putting the cliffs
at a layer on top with the Color Erase blending mode.

Although this PNG can be improved in GIMP or Aseprite, it is not
worth the effort. It would be better to gain access to the source
artwork.

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Port Fray's End to new elevation tileset.

Change the tileset resource in the 3 elevation TileMapLayer
nodes. Remove the now missing wall tiles. Add the alternative
collision-less tile behind the two bridges.

Also add a new 2 TileMapLayer for decorations:

• On level 0: a mine entry.
• On level 1: a staircase and a mine entry blocked by rocks.

Resolve #1613

[manuq]

The cliffs at the east of Fray's End has been fixed by this. Before/after:

[github-actions]

Play this branch at https://play.threadbare.game/branches/endlessm/simple-elevations.

(This launches the game from the start, not directly at the change(s) in this pull request.)

[wjt]

Looks good!