fix: declare vPosition input in fragment shader to prevent panic

shaders/simple.frag

@@ -3,6 +3,7 @@
 layout(location = 0) out vec4 color;
 in vec4 vColor;
 in vec3 vNormal;
+in vec3 vPosition;
 
 void main() {
     vec3 lightDirection = normalize(vec3(0.8, -0.5, 0.6));