Norm16 texture formats

Adds support for norm16 formats with CopySrc|CopyDest|RenderAttachment|TextureBinding usages, multisampling and resolving capabilities. Additional formats are:

• wgpu::TextureFormat::R16Snorm
• wgpu::TextureFormat::RG16Snorm
• wgpu::TextureFormat::RGBA16Snorm
• wgpu::TextureFormat::R16Unorm
• wgpu::TextureFormat::RG16Unorm
• wgpu::TextureFormat::RGBA16Unorm

The initial tracking bug was https://crbug.com/dawn/1982.