implemented demonstrative dip to black transition

app/shaders/diptoblack.frag

@@ -0,0 +1,30 @@
+#version 110
+
+varying vec2 ove_texcoord;
+
+uniform sampler2D out_block_in;
+uniform sampler2D in_block_in;
+uniform bool out_block_in_enabled;
+uniform bool in_block_in_enabled;
+
+uniform float ove_tprog_out;
+uniform float ove_tprog_in;
+
+void main(void) {
+  vec4 out_block_col;
+  vec4 in_block_col;
+
+  if (out_block_in_enabled) {
+    out_block_col = texture2D(out_block_in, ove_texcoord) * pow(ove_tprog_out, 2.0);
+  } else {
+    out_block_col = vec4(0.0);
+  }
+
+  if (in_block_in_enabled) {
+    in_block_col = texture2D(in_block_in, ove_texcoord) * pow(ove_tprog_in, 2.0);
+  } else {
+    in_block_col = vec4(0.0);
+  }
+  
+  gl_FragColor = out_block_col + in_block_col;
+}