WIP anvil/udev: Dynamic support for pixman rendererer

[ids1024]

With the changes here, the anvil udev backend will use pixman for rendering if ANVIL_USE_PIXMAN is set.

To do:

• Figure out how to deal with Send bound on MemoryRenderBuffer, and no Send for PixmanTexture
• Fix build with debug feature (need to abstract texture)
• Make render node optional, and automatically use pixman if hardware acceleration isn't present
• Pixman support in x11/winit backends

It's not entirely clear how best to abstract renderers dynamically:

• Should an enum wrapper implement the Renderer trait?
  • How does that impact performance or binary size compare to dispatching to a render function twice like this currently does?
  • Will also need enums over texture, frame, error; and probably a panic if these types are mismatched in some way
• Would any changes to Renderer and related traits make it easier to abstract over, and dispatch at runtime to different renderers?

[cmeissl]

I tried to create a wrapper enum implementing the renderer traits. Bind is the most "problematic" as this can be different per renderer.

Macro based implementation: https://github.com/cmeissl/smithay/blob/3834756520cfb8db5c21b9b5485d16bdc7cdab6e/src/backend/renderer/auto.rs
Usage of the enum: https://github.com/cmeissl/smithay/blob/3834756520cfb8db5c21b9b5485d16bdc7cdab6e/anvil/src/render.rs#L28

[ids1024]

I was thinking a macro might make sense. That looks handy. Although macro rules macros like that get a bit complicated.