fix(physics): update local body variable after creating MatterTileBody

[cyphercodes]

Description

Fixes a scope bug in MatterTileBody where the local 'body' variable remained null after creating the body via setFromTileCollision() or setFromTileRectangle(). This caused a crash when processing flipped tiles because Body.scale() was called with null.

Bug

When Matter.World.convertTilemapLayer() processes a tile that has flipX or flipY set, it crashes with:

Root Cause

The constructor declares a local body variable initialised to null, then creates the actual body via setFromTileCollision() or setFromTileRectangle() — both of which set this.body but never update the local body variable. Immediately after, the flip check reads from the stale local.

Fix

Add one line to update the local body variable after the body is created:

Fixes #7267

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Note

Low Risk
Low risk, single-line fix in MatterTileBody constructor to prevent null body usage when scaling flipped tiles.

Overview
Fixes a constructor scoping bug in MatterTileBody where the local body variable could remain null after creating a body from tile collision shapes or a tile rectangle.

After generating the body via setFromTileCollision() / setFromTileRectangle(), it now reassigns body = this.body so the subsequent flip handling (Body.scale for flipX/flipY) operates on the created body instead of crashing.

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