System Information
Win 7
I7-5820K
GTX780 ti (372.54)

Blender Version
Broken: 2.78.4 67bef6b
Worked: 2.78 e8299c8 (official)

Short description of error
The background image is not showing in viewport (but it's rendering alright) if I use a lightpath as a factor to mix two environment images texture. A message appear in the windows console :
GPUShader: compile error:
0(3699) : error C1104: too many parameters in function call

This is not happening with the official.

Exact steps for others to reproduce the error
Open the .blend file : No background image appear in the viewport (if you render, it's ok), look in the window console for a error message.

Unlink the lightpath from the mix shader, the background image appear in the viewport.

Thanks a lot for your work !