Implement being able to drop through soft platforms (#64)

Reviewed-on: #64
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/stage/line.rs

@@ -34,7 +34,7 @@ pub struct StageLineID {
 
 #[derive(Reflect, Clone, Default)]
 pub struct StagePoly {
-    poly_type: StagePolyType,
+    pub poly_type: StagePolyType,
     pub segments: Vec<StagePolyLineSegment>,
 }