chore: import upstream snapshot with attribution

src/shaders/debug-shader.ts

@@ -0,0 +1,37 @@
+const vertexShader = /* glsl */ `
+    attribute vec3 vertex_position;
+    attribute vec4 vertex_color;
+
+    varying vec4 vColor;
+    varying vec2 vZW;
+
+    uniform mat4 matrix_model;
+    uniform mat4 matrix_viewProjection;
+
+    void main(void) {
+        gl_Position = matrix_viewProjection * matrix_model * vec4(vertex_position, 1.0);
+
+        // store z/w for later use in fragment shader
+        vColor = vertex_color;
+        vZW = gl_Position.zw;
+
+        // disable depth clipping
+        gl_Position.z = 0.0;
+    }
+`;
+
+const fragmentShader = /* glsl */ `
+    precision highp float;
+
+    varying vec4 vColor;
+    varying vec2 vZW;
+
+    void main(void) {
+        gl_FragColor = vColor;
+
+        // clamp depth in Z to [0, 1] range
+        gl_FragDepth = max(0.0, min(1.0, (vZW.x / vZW.y + 1.0) * 0.5));
+    }
+`;
+
+export { vertexShader, fragmentShader };