ENH: refine the thumbnail rendering

1. add specific shader for thumbnail
2. do not render the negative-z part
3. fix the sinking related issue under cli

Change-Id: Ia59083437544c1c0bd924c811274ccbb137e9eb7

resources/shaders/thumbnail.fs

@@ -0,0 +1,16 @@
+#version 110
+
+uniform vec4 uniform_color;
+uniform float emission_factor;
+
+// x = tainted, y = specular;
+varying vec2 intensity;
+//varying float drop;
+varying vec4 world_pos;
+
+void main()
+{
+    if (world_pos.z < 0.0)
+        discard;
+    gl_FragColor = vec4(vec3(intensity.y) + uniform_color.rgb * (intensity.x + emission_factor), uniform_color.a);
+}