[Impeller] Deprecate and remove Texture::SetContents in favor of BlitPass.

[jonahwilliams]

This API has a few problems, namely:

• It exists at the core layer, before command buffers are defined. But encoding/uploading safely requires a cmd buffer object on Vulkan and Metal. Vulkan works around this by internally creating a cmd buffer and performing a copy to a VMA buffer, but this is extra wasteful.
• The metal implementation uses an unsafe API.
• It duplicates functionality that already mostly exists in the BlitPass.

Ripping out in one go is too difficult though, as its used all over the place and exposed via flutter gpu.

[jonahwilliams]

Work started here: flutter/engine#52510