Support transient attachments on Vulkan and Metal

[opstic]

Is your feature request related to a problem? Please describe.
As it appears suggested by multiple mobile vendors (Arm, Qualcomm, etc), the optimal way to resolve multisampling is by using a transient attachment and doing it in tile memory only. It would much reduce the bandwidth needed for resolve operations and reduce memory usage.

Describe the solution you'd like
Implement a Feature::TRANSIENT_ATTACHMENT and TextureUsages::TRANSIENT like Dawn maybe

Additional context
Arm documentation on this matter for Vulkan
Corresponding gpu_alloc::UsageFlags::Transient
Apple documentation on MTLStorageMode.memoryless

Also maybe have an implementation in OpenGL for efficient resolve operations with EXT_multisampled_render_to_texture?
Arm documentation on this matter for OpenGL
Corresponding Qualcomm code example

[cwfitzgerald]

Yeah we definitely want this, and there's movement to get this in the spec as well.