Create xr full screen effects doc page

[devloglogan]

Adds a doc page informing on how users can take asymmetric FOV into account in full screen quad post-processing shaders.

@BastiaanOlij mentioned that attempting to do this while reading from the screen texture is a big no-no so I also included a section at the bottom attempting to warn users about that.

[BastiaanOlij]

Personally i feel we shouldn't be talking about post processing here. Post processing effects suggest that you modify the rendered output and that is where the big no-no's come in.

Vignettes' etc are not post processes. They are screen space effects that use a quad to render their output (actually a single triangle would be better but we don't have an easy primitive for that). So I think we need to come up with better naming.

In the same line, I don't think that referencing the original post processing document is a good idea. Again, we just set people on the wrong footing, they will read that, figure out they can do that in XR as well, and then when they are too far along find out that doing so is what is causing their performance to never get up to spec. By then it's too late.

[devloglogan]

@BastiaanOlij understood, and now that I understand the term a bit better I fully agree! @dsnopek had mentioned something similar to me elsewhere. I'll rework this soon to swap that term out for something like "full screen effects" and also remove the reference to the other doc.

[BastiaanOlij]

@devloglogan it does feel like nitpicking but I feel we can prevent ourselves a bit of hardship if we get that bit right :)

[devloglogan]

Alright, I've opted for "full screen effects" as the term instead and removed the reference to the other doc entirely (which adds a touch more on the basic full screen quad setup).

[devloglogan]

@dsnopek Thanks for the review! I believe I've addressed all of your suggestions, please take another look at the doc when you get a chance. :)

[dsnopek]

Thanks! This is looking good to me now :-)

[skyace65]

Grammer and spelling should be good after these two changes. @BastiaanOlij did you want to look this over again?

[BastiaanOlij]

LGTM, nice writeup Logan! Indeed just need the small grammar fixes and its ready to merge IMHO.

[skyace65]

Thanks!