Fix bug in WorldHeightMap::Parse_Blend_Tile_Data function

[xezon]

• Split off of #796

This is addressing supposed original game bug.

tomsons26

indeed it's a bug in original, not too sure if this should be accepted, don't know the implications of fixing this..

I unclear what this does. is_blend_edge_tile member is currently not used in Thyme.

Also a few lines lower is another file read and that also does not set is_blend_edge_tile.

Because is_blend_edge_tile is a legacy thing that would need to be removed entirely.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (cfa005f) 2.53% compared to head (f76a1da) 2.53%.

Files	Patch %	Lines
src/platform/w3dengine/client/worldheightmap.cpp	0.00%	4 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1112 +/- ## ======================================== Coverage 2.53% 2.53% ======================================== Files 949 949 Lines 110299 110299 Branches 18881 18881 ======================================== Hits 2800 2800 Misses 107095 107095 Partials 404 404 ```

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