MultiEffect source item strategy

Consider how MultiEffect would handle its source item, how it would be both efficient and easy to use.

1) Always create ShaderEffectSource. (currently)

• Pros: Just works
• Cons: When source already is textureProvider this wouldn't be needed, so creates an extra texture.

2) Never create ShaderEffectSource. (GE)

• Pros: Never extra texture.
• Cons: User needs to make sure source is texture provider (use ShaderEffectSource or set layer.enabled: true)

3) Create ShaderEffectSource when needed, check isTextureProvider() and item.layer.enabled

• Pros: Just works, texture only when needed.
• Cons: Does this actually work?

4) Set automatically layer.enabled for the source.

• Pros: Just works, texture only when needed.
• Cons: Is it good that effect changes property of source item?

5) Have "bool createSource" property.

• Pros: Allows controlling source need.
• Cons: Would default be true or false? Is is any better than requiring layer.enabled?