GPU examples

[Giesch]

This adds / will add some more SDL GPU examples based on the official ones in C.

Notes:

• the shader json metadata is from shadercross
  I've been using a justfile setup for managing that in my own project, we could do something similar here,
  or I could remove the serde dependencies and hardcode the resource counts.
  The serde dependency is also the reason for the new examples directory (based on axum's examples setup).
• my editor/LSP setup locally was sad without a rustfmt.toml specifying the edition

Remaining todos:

• use some more rust-friendly patterns (ie avoid unnecessary SDL_free)
• re-add the clear screen example
• re-add the basic triangle example
• find a way to share the common module
• maybe add a shadercross setup for compiling example shaders
• maybe switch examples to using app_impl
• remove build-from-source-static

[Lokathor]

if there should be a rustfmt file just add a rustfmt file

[Giesch]

Sorry; I can see how that'd come off as catty. I meant it as an explanation for a kind of pointless change. I don't think the file's actually required by rustfmt itself, only by my editor extension.

[Lokathor]

I'm not the lead but the crates are using 2021 edition, so there should probably be a rustfmt that configures for that.

[maia-s]

Looks good so far! Thank you for doing this.

I don't think the examples should require the build-from-source-static feature so people can run them with an already installed SDL library, but you can leave taking that out for last if it makes developing easier for you.

[maia-s]

The examples fail here (macos) with newLibraryWithSource:8657: failed assertion `source must not be nil.'. I can investigate later.