In this part we turn our passthrough material into a grayscale material by modifying the fragment shader.
We update window.fp with the following grayscale formula from Wikipedia.
varying mediump vec2 var_texcoord0;
uniform lowp sampler2D texture_sampler;
void main()
{
vec4 color = texture2D(texture_sampler, var_texcoord0.xy);
float grayscale = dot(color.rgb, vec3(0.299, 0.587, 0.114));
gl_FragColor = vec4(grayscale, grayscale, grayscale, color.a);
}