Fix Render2D line thickness scaling

Use transformed 2D basis vector lengths instead of diagonal matrix entries when scaling line thickness. This keeps Render2D.DrawLine and rectangle outlines stable under rotation while preserving scale transforms.

Fixes #4140.

Source/Engine/Render2D/Render2D.cpp

@@ -247,6 +247,13 @@ FORCE_INLINE void ApplyTransform(const Float2& value, Float2& result)
     Matrix3x3::Transform2DPoint(value, TransformCached, result);
 }
 
+FORCE_INLINE float GetTransformScale()
+{
+    const Float2 axisX(TransformCached.M11, TransformCached.M12);
+    const Float2 axisY(TransformCached.M21, TransformCached.M22);
+    return (axisX.Length() + axisY.Length()) * 0.5f;
+}
+
 void ApplyTransform(const Rectangle& value, RotatedRectangle& result)
 {
     const RotatedRectangle rotated(value);
@@ -1464,7 +1471,7 @@ void Render2D::DrawRectangle(const Rectangle& rect, const Color& color1, const C
 
     const auto& mask = ClipLayersStack.Peek().Mask;
     float thick = thickness;
-    thickness *= (TransformCached.M11 + TransformCached.M22 + TransformCached.M33) * 0.3333333f;
+    thickness *= GetTransformScale();
 
     // When lines thickness is very large, don't use corner caps and place line ends to not overlap
     if (thickness > 4.0f)
@@ -1840,7 +1847,7 @@ void DrawLines(const Float2* points, int32 pointsCount, const Color& color1, con
     ASSERT(points && pointsCount >= 2);
     const auto& mask = ClipLayersStack.Peek().Mask;
 
-    thickness *= (TransformCached.M11 + TransformCached.M22 + TransformCached.M33) * 0.3333333f;
+    thickness *= GetTransformScale();
 
     Render2DDrawCall& drawCall = DrawCalls.AddOne();
     drawCall.StartIB = IBIndex;