Fix #167

While researching what causes #1193 (closed) I found out that this is actually the same problem as #167. That texture used to render properly before the unification of the texture and flat managers, but now it maps to texture ID 0 always. This PR just makes -1 the no-texture-ID instead of 0.

changed milestone to %2.2.14

added Bug Changed behavior Refactoring Rendering labels

added 1 commit

• 76d353ff - Log when a level flat isn't found

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changed the description

changed milestone to %2.2.15

added 28 commits

• 76d353ff...6d645acb - 27 commits from branch next
• 5119ec89 - Merge branch 'next' into fix-167

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mentioned in merge request !1738 (closed)

added Unmergeable label

requested review from @LoganAir

assigned to @LoganAir

changed milestone to %2.2.16