Custom sky textures are ignored when deferred lighting is enabled

Custom skyboxes in texture packs are incompatible with the deferred lighting rendering pipeline.

Steps to reproduce:

• Apply any deferred lighting-enabled texture pack that has a custom sky (or apply the provided example pack).
• Enable deferred lighting.
• Observe that the custom sky cubemap disappears when deferred lighting is enabled.

Observed behavior: the custom sky will render only when deferred lighting is disabled.

Expected behavior: the custom sky textures should be overlaid on the sky whether or not deferred lighting is enabled.