library: core: add LinearColor type

src/core/color.rs

@@ -0,0 +1,6 @@
+#[derive(Debug)]
+pub struct LinearColor {
+    pub r: f64,
+    pub g: f64,
+    pub b: f64,
+}

src/core/mod.rs

@@ -0,0 +1,2 @@
+pub mod color;
+pub use color::*;