feat: switch to gltf crate and remove easy_gltf

This is because easy_gltf did not expose texture information, which the gltf crate did.

src/common/material.rs

@@ -0,0 +1,36 @@
+use std::collections::HashMap;
+
+pub struct MaterialRegistry {
+    materials: HashMap<String, Material>
+}
+
+impl MaterialRegistry {
+    pub fn new() -> Self {
+        Self {
+            materials: HashMap::new()
+        }
+    }
+
+    pub fn register(&mut self, id: String, material: Material) {
+        self.materials.insert(id, material);
+    }
+
+    pub fn get(&self, id: &String) -> Option<&Material> {
+        self.materials.get(id)
+    }
+}
+
+/// Encapsulates all the information that makes up a PBR (physically based rendering) material.
+/// These materials should be stored in a [`MaterialRegistry`], with a unique key.
+pub struct Material {
+    /// The base color of the material.
+    pub base_color: [f32; 4],
+    /// The metallic value of the material. This should be a number between 0 and 1.
+    pub metallic: f32,
+    /// The roughness value of the material. This should be a number between 0 and 1.
+    pub roughness: f32,
+    /// The path of the normal map for this material, if any.
+    pub normal_map: Option<String>,
+    /// The path of the base color's texture, if any.
+    pub base_color_texture: Option<String>,
+}