[Graphics] Order-independent transparency support

Release Type: Both

Version: Future

Platform(s): Likely only supportable on Desktop

Describe the feature
Currently, Xenko performs a somewhat naive alpha blend for transparency. This isn't necessarily bad, but for proper rendering of transparency for things like particles or geometry that intersects or overlaps (such as glassware), some form of order-independent transparency would be nice.

Additional context
I'm not a graphics programmer, so I'm definitely not the best person to take up the banner on this feature.

However, since the only other public implementation (Amazon Lumberyard) requires hardware support for Direct3D12 while there are versions of this technique that don't (hell, even the old reliable dual depth peel was originally shown by Nvidia years before Direct3D12 was even announced), this would be "easy" (using this loosely as it would no doubt require some modifications to the renderer and compositor) to support this under Direct3D11 instead.

The issue names no files, tests, or entry points. Start by locating the renderer and compositor and reviewing desktop transparency support, including the Direct3D11 constraints described here. Done would require an agreed order-independent transparency approach and verified support for the stated transparency cases.