s3db: disable postprocess mode

- seems old mali having issues with dependent texture reads..
- add bypass shader for testing

vtm/resources/assets/shaders/post_bypass.glsl

@@ -0,0 +1,28 @@
+#ifdef GLES
+precision highp float;
+#endif
+uniform vec2 u_pixel;
+attribute vec4 a_pos;
+varying vec2 tex_pos;
+
+void main()
+{
+  gl_Position = a_pos;
+  tex_pos = (a_pos.xy + 1.0) * 0.5;
+}
+
+$$
+
+#ifdef GLES
+precision highp float;
+#endif
+uniform sampler2D u_texColor;
+uniform vec2 u_pixel;
+varying vec2 tex_pos;
+
+void main(){
+  
+  gl_FragColor = texture2D(u_texColor, tex_pos) * 0.8;
+}
+
+