The `image_get_gpu_texture` didn't use the iuser->view_index but
recalculated the requested view again. This lead to inconsistent
behavior when switching between multi view textures or stereo textures.
This has been fixed by ensuring that the `iuser->view_index` is always
used.
An Image has only place to store 2 view textures. This is done for
right/left eye compositing. A multi view texture can have more views.
This would lead to reading and writing to unallocated space.
When a multiview texture is requested that is larger than 1. It will
always be cached as being the first eye. The `gpu_view` of the Image is
also used as a cache key to check this.
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