Implement Convert trait to convert between CPU and GPU images (#3194)

* Add upload texture trait

* Make convert trait use explicit converter

* Add gpu texture download implementation

* Add footprint to convert trait

* Cleanup texture upload / download

* Download wgpu textures aligned

* abstract texture download into converter helper

* rename module

not only doing uploads anymore
conversion looks like a ok name

* Remove into_iter call and intermediate vector allocation

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Co-authored-by: Timon Schelling <me@timon.zip>

node-graph/wgpu-executor/src/lib.rs

@@ -1,6 +1,6 @@
 mod context;
 pub mod shader_runtime;
-pub mod texture_upload;
+pub mod texture_conversion;
 
 use crate::shader_runtime::ShaderRuntime;
 use anyhow::Result;