Title screen panorama uses new textures even when any resource pack with custom background is used

Relates to MC-140210.

The issue

If a resource pack with custom background is used, the panorama in the background would be expected to show a scene utilizing said textures. This is not the case.

How to reproduce

Download and install the pack made by me.
(Link: https://drive.google.com/open?id=1Ds4S-nLJwLT92Jo782-dFlpjIumwaeK5)