Textures that are used as uniform variable value in a ShaderEffect don't get disposed with the ShaderEffect #867

FUSEE v0.13.0

Textures that are used in a ShaderEffect are kept alive by the TextureManager, even if the correpsonding ShaderEffect is already disposed.
Textures are registered in the TextureManager on creation. From then on, the TextureManager holds a reference to the Texture object.
But there is no connection from the ShaderEffect/EffectManager to the TextureManager. Therefore the TextureManager will keep the reference to the texture indefinitely.
A workaround is to manually delete the Texture when we know that the ShaderEffect is no longer needed. However, this isn't ideal or user-expected behavior.