draw overlay hover bar

assets/fs/shaders/overlay_hover.f.glsl

@@ -0,0 +1,18 @@
+
+varying vec3 pos_i;
+
+const vec4 red = vec4(1, 0, 0, 1);
+const vec4 yellow = vec4(1, 1, 0, 1);
+const vec4 green = vec4(0, 1, 0, 1);
+
+void main(void)
+{
+    if (pos_i.x < 0)
+    {
+        gl_FragColor = mix(red, yellow, pos_i.x + 1);
+    }
+    else
+    {
+        gl_FragColor = mix(yellow, green, pos_i.x);
+    }
+}