Fixed NativeEngine render target assignment

[SergioRZMasson]

Native engine is currently not assingning the new texture passed in bindFrameBuffer to its _currentRenderTarget. This will result in NaN values for camera's projection matrix since it uses the engine.getRenderWidth() and engine.getRenderHeight() and those values will be zero.

Storing _currentRenderTarget fixes the problem.

[azure-pipelines]

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[azure-pipelines]

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[Popov72]

You should also set it to null in unBindFramebuffer.

[SergioRZMasson]

@Popov72 , updated PR with changes.