Lenticular shader should support texture arrays

[mcclure]

When rendering a scene from multiple angles at once (as is common in VR, and needed for Holoplay) there are three basic options:

1. Render the scene multiple times.
2. Do an instanced render to a single texture, and assign different viewports for each instance.
3. Do an instanced render, and render to texture arrays.

(2) and (3) can be a significant performance improvement in OpenGL over (1).

However, (2) is for various reasons problematic. In particular (2) is not available on all OS/hardware configurations (for example if I'm not mistaken (3) is available on Apple OpenGL but not (2), because texture arrays were added in OpenGL 3 but glViewportArray did not become core until OpenGL 4).

Holoplay SDK ingests a "quilt" grid of rendered images, which limits your options to (1) or (2). [Yes, you could also render to a texture array and then blit the texture array to a quilt, but this would require an additional bandwidth-intensive step per frame.] However, the "ingestion" is done by the lenticular shader, and the lenticular shader doesn't seem to be very long.

"Expected behavior": I think it would be pretty easy to alter the lenticular shader so that rather than taking its source images from a quilt, it took the source images from a texture array. But it would probably be necessary for Looking Glass to do this modification, because (due to the IP restrictions on the lenticular shader) a third party would not be able to redistribute the modified source. This would make it easier to support Holoplay from engines which are already based around texture arrays.