[Metal] Better creation error tracking

[kvark]

Sadly, Metal is not supported on my old Mac Mini 2010.
If someone got a newer one, please try running our examples with "--feature metal" flag. It would be nice to publish that Metal backend at last.

[doppioslash]

@kvark:
On a 10.12.2 Macbook Pro from 2015:

• triangle, gamma, and shadow succeed
• cube, deferred, flowmap and terrain fail with:

thread 'main' panicked at 'not yet implemented', src/backend/metal/src/command.rs:303

• blend, particle, mipmap, ubo_tilemap fail with:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: SelectError(Msl(11))', ../src/libcore/result.rs:799

• skybox crashes with:

-[MTLTextureDescriptorInternal validateWithDevice:]:774: failed assertion `MTLTextureDescriptor width must equal height for cube textures.'

• terrain_tessellated fails with:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Hull(StageNotSupported(Hull))', ../src/libcore/result.rs:799

• instancing fails with:

32 instances per length
1024 instances
size: 0.05
thread 'main' panicked at 'not yet implemented', src/backend/metal/src/factory.rs:760

[kvark]

@doppioslash incredible, thank you! We'll revisit this comment many times for sure when implementing missing features. Please feel free to tackle them as well if you want!
@homu r+

[homu]

📌 Commit cbc9c8a has been approved by kvark

[homu]

⌛ Testing commit cbc9c8a with merge ab4c921...

[kvark]

triangle, gamma, and shadow succeed

Triangle and Gamma use glutin directly. Shadow example defaults to GL.
So it appears that Metal is no-functional at the moment...

[Bastacyclop]

@kvark This is probably partially because of this from recent PRs: https://github.com/gfx-rs/gfx/blob/master/src/backend/metal/src/command.rs#L303:

1. There is an usage redundance used in buffer updates just like the one I removed in DX11 (Vulkan may have it too).
2. map_buffer_usage now requires Bind too, so the latter is not enough to call it anymore.

However buffer updates are currently Dynamic only so things could be hardcoded for it.

[kvark]

@Bastacyclop thanks for the info!
It would be really great to have someone picking up the Metal backend, given @fkaa 's absence. I wouldn't mind maintaining it myself, but my Mac is too old. Help wanted!

[homu]

☀️ Test successful - status