vello_hybrid: Add render_to_atlas and write_to_atlas APIs for glyph caching

[grebmeg]

Adds public APIs to both the wgpu and webgl renderers for rendering Scene content directly into atlas layers and writing pixel data to pre-allocated atlas regions. This enables a glyph caching workflow where vector glyphs are rasterized into the atlas once and reused as cached images in subsequent passes.

The key additions are render_to_atlas, which runs the scheduler targeting a specific atlas layer (using a placeholder dummy texture to avoid read-write hazards), and write_to_atlas, extracted from upload_image, which writes pixel data to an already-allocated region.

[LaurenzV]

Hmm... What I did is to change add an OutputTarget that either refers to the final view or an intermediate texture (

vello/sparse_strips/vello_hybrid/src/schedule.rs

Lines 211 to 230 in b23b634

	/// The output target for the final composite draw in a round.
	///
	/// This distinguishes between rendering to the user-visible surface and
	/// rendering to an intermediate texture for a filter layer.
	#[derive(Debug, Clone, Copy, PartialEq, Eq)]
	pub(crate) enum OutputTarget {
	/// Render to the final output view/surface.
	FinalView,
	/// Render to an intermediate texture for a layer with filter effects.
	IntermediateTexture(LayerId),
	}
	/// Specifies the target for a render pass.
	#[derive(Debug, Clone, Copy, PartialEq, Eq)]
	pub(crate) enum RenderTarget {
	/// Render to the final output or an intermediate texture.
	Output(OutputTarget),
	/// Render to one of the slot textures used for clipping/blending.
	SlotTexture(u8),
	}

), and then the selection can happen at runtime instead of having to duplicate the whole render method, would that work for you? Otherwise, seems fine at a glance, but it might be worth extracting all of the setup stuff (e.g. for encoded paints) into a wrapper function that does setup/teardown and takes a closure for the "main action". What do you think?