Use raw-window-metal to do layer creation

[madsmtm]

The logic for creating the CAMetalLayer if it doesn't exist is complex, and currently Slint is doing it wrong and prevents Winit from scheduling redraw events at the correct time. I have abstracted this away into raw-window-metal, it works by creating a layer, and inserting that as a sublayer. The bounds and scale factor are then kept in-sync with an observer, ensuring smooth resizing. This is also important for iOS support, since overwriting the layer, as currently done, isn't possible there.

See also corresponding PR to wgpu, PR to ash and PR to vulkano for more details and reasoning.

I'm pretty sure that you can remove Surface::set_scale_factor after this, it seems to have only been used for MetalSurface, but it would be a breaking change so I'm inclined to let someone else do it later.

Note also the change from setting layerContentsPlacement to contentsGravity, this should be a non-functional change, since we do as the documentation suggests, and check contentsAreFlipped first, but I haven't tested it enough to be certain.

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[tronical]

Thank you! Just last week I observed your new crate and was going to do exactly what you suggested. I've also made the same observation with iOS.