Sharing Vulkan memory object

**Is your feature request related to a problem? Please describe.**
I need to render to a GL Texture external to Filament, while using Vulkan backend.

**Describe the solution you'd like**
Have RenderTarget share Vulkan memory object, so external to Filament I can implement GL interop code. As the GL memory object needs to be created from a Vulkan memory object.

**Describe alternatives you've considered**
Modified Filament to run on Linux EGL/GLESv2 (1.5/3.2) backend. Hitting issue where Wayland does not support pbuffer. Was asked by @pixelflinger if Vulkan was an option.

**OS and backend**
Linux + Vulkan backend

Useful Vulkan/GL interop patterns documented here:
https://eleni.mutantstargoat.com/hikiko/vk-gl-interop-2/
https://eleni.mutantstargoat.com/hikiko/vk-gl-interop-3/