[mtl] clear_attachments changes viewport but does not reset it

[zakorgy]

Short info header:

• GFX version: hal-0.4
• OS: MacOS
• GPU: Intel

Calling clear_attcahment after the viewport is set changes the viewport but does not reset it.
I've a modified quad example for it: https://github.com/zakorgy/gfx/tree/clear-attachment (just added a clear_attachment to clear an area to red)

The expected result with Vulkan and DX12:

The actual result with Metal, the rendered image is flipped:

I suspect this is caused by changing the viewport in clear_attachments: https://github.com/gfx-rs/gfx/blob/hal-0.4/src/backend/metal/src/command.rs#L2953 . Also I'm not sure what happens with scissor, because it's also set for the clear but doesn't seem to affect the result.

cc @kvark