Select supported alpha mode based on surface capabilities

[phideg]

Issue

CompositeAlphaMode::PostMultiplied was used unconditionally to configure a wgpu::Surface.

Requesting an unsupported alpha mode prevents applications from initializing the surface on some platforms/adapters and produces following runtime validation error.

Requested alpha mode `PostMultiplied` is not in the list of supported alpha modes: [...]

Solution

Query surface.get_capabilities(&adapter).alpha_modes and choose a supported CompositeAlphaMode from this preference order:

• PostMultiplied
• PreMultiplied
• Opaque (fallback)

[bvssvni]

Merging.

[bvssvni]

Thanks!