Render: add backend capability discovery and strict request resolution (#650)

Adds render.backends, a metadata-only command that lists the render backends, their executors and profiles, with strict request resolution in the render crate (docs/render-backends.md).

crates/render/src/lib.rs

@@ -4,10 +4,16 @@
 #![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 #![forbid(unsafe_code)]
 
+mod backend;
 mod camera;
 mod curve_display;
 mod raster;
 
+pub use backend::{
+    Availability, BackendDescriptor, BackendId, ExecutorDescriptor, ExecutorId, MAX_RENDER_ID_BYTES, RenderFeature, RenderFormat, RenderLimits,
+    RenderMode, RenderOperation, RenderPolicy, RenderProfile, RenderRequest, ResolveError, ResolvedRequest, ScratchLayout, builtin_backend,
+    resolve_request,
+};
 pub use camera::{Camera, CameraAnim, Mat4, Quat, StandardView, UpAxis, ease_in_out};
 pub use curve_display::{PixelCurveView, SampledPath, SamplingBudget, SamplingQuality, sample_segment, sample_segment_unclipped};
 pub use raster::{Canvas, Rgb, SECTION_CAP, Scene, SceneLine, SceneMesh, png, render, render_png, render_transparent};