Color system based on traits, and conversion to linear color in the graph (#1123)

* Migrate Nodes to use RasterMut + Samplable

* Add Pixel trait to include serialization

* Implement traits for Color and propagate new generics

* Always convert to linear color when loading images

Cargo.lock

@@ -1595,6 +1595,9 @@ dependencies = [
  "kurbo",
  "log",
  "node-macro",
+ "num",
+ "num-derive",
+ "num-traits",
  "once_cell",
  "rand_chacha 0.3.1",
  "serde",
@@ -2657,6 +2660,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "num-derive"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "876a53fff98e03a936a674b29568b0e605f06b29372c2489ff4de23f1949743d"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+]
+
 [[package]]
 name = "num-integer"
 version = "0.1.45"