Remove 45 degree uv rotation from the shader as it's not needed anymore.
Bring back the stretched perspective, because the background was
designed with it in mind, and looks wrong otherwise.
this means glm_mat_proj_perspective is no longer meant to be called by the stage code. The parameters are set to sensible defaults of the pbr stages.
I found that the old stages use STAGE3D_DEFAULT_ASPECT which is somehow stretched. I did not change the behavior but added a FIXME comment to lines that should at some point just be deleted to use the correct default aspect.
I did not fix the extra stage code to avoid conflicts.
