Tile animation marker gets in the way of Terrain labels

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### Problem
When editing Terrains in a tileset that has animations defined, the animation marker in the corner of the tiles can get in the way, even though it's drawn below the labels - it obscures the tile art and makes it harder to know what the label should be, and in some cases, it can be hard to tell whether there's a label there at all.

### Proposed solution
It would be nice if, when the Terrain Sets editor is open, the animation markers were drawn with a lower opacity, so that the tile art beneath is visible, and so that they don't overwhelm low-contrast terrain labels.
Hiding them entirely would not be good, as they're often useful to know which tiles should get terrain labels and which should not.

Start in the Terrain Sets editor and locate where tile animation markers are drawn while terrain labels are visible. Verify the marker appearance in an animated tileset, then confirm that reduced opacity keeps the tile art and terrain labels readable without hiding the marker.

Written by the indexing model from the issue text.