load shaders from files.

vis/shaders/one-color.fs.glsl

@@ -0,0 +1,12 @@
+#version 430
+
+layout(location=2) uniform vec3 c;
+
+in vec4 vpos;
+
+out vec4 color;
+
+void main() {
+    float d = smoothstep(-2, 2, vpos.z);
+    color = vec4(c * d, 1);
+}