Implement Infrastructure to reuse previous frames for brush drawing

Implement Infrastructuro to reuse the previous evaluation of the
node graph to blend the new stroke with instead of drawing the
entire trace from scratch.
This does not transition to a blending based approach because that still
caused regressions but allows the brush node to work with input data
natively.

Test Plan:
- Use the brush tool in the editor and check for regressions
- Evaluate the performance

Reviewers: Keavon

Pull Request: https://github.com/GraphiteEditor/Graphite/pull/1190

node-graph/gcore/src/lib.rs

@@ -39,8 +39,9 @@ pub trait Node<'i, Input: 'i>: 'i {
 	type Output: 'i;
 	fn eval(&'i self, input: Input) -> Self::Output;
 	fn reset(self: Pin<&mut Self>) {}
-	#[cfg(feature = "alloc")]
-	fn serialize(&self) -> Option<String> {
+	#[cfg(feature = "std")]
+	fn serialize(&self) -> Option<std::sync::Arc<dyn core::any::Any>> {
+		log::warn!("Node::serialize not implemented for {}", core::any::type_name::<Self>());
 		None
 	}
 }

node-graph/gcore/src/raster.rs

@@ -139,6 +139,7 @@ pub trait UnassociatedAlpha: RGB + Alpha {
 
 pub trait Alpha {
 	type AlphaChannel: Channel;
+	const TRANSPARENT: Self;
 	fn alpha(&self) -> Self::AlphaChannel;
 	fn a(&self) -> Self::AlphaChannel {
 		self.alpha()

node-graph/gcore/src/raster/color.rs

@@ -96,6 +96,8 @@ impl Pixel for Color {
 
 impl Alpha for Color {
 	type AlphaChannel = f32;
+	const TRANSPARENT: Self = Self::TRANSPARENT;
+
 	fn alpha(&self) -> f32 {
 		self.alpha
 	}