[mtl] use existing GfxManagedMetalLayerDelegate if available

[kvark]

Fixes https://github.com/gfx-rs/gfx/pull/3579#issuecomment-759497638 PR checklist:

• [x] make succeeds (on *nix)
• [ ] make reftests succeeds
• [x] tested examples with the following backends: Metal

[kvark]

bors r+

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable