Joannesalfa
First of all, it is a good idea to have a look into the shaders.js when you get errors. In chrome you can do this by pressing ctrl+shift+i. Go to the tab named "Sources", then "shaders.js". The specific error you encountered is present at Line 19, because the first line just initialzes the following code for C2.
"Varying" variables can't be altered at runtime, so therefore you get that error. To bypass it you can simply define a new vec2 variable after "void main( void ) {" like this:
vec2 myPos = position;
then just replace the following "position" variable with "myPos".
One further suggestion when copy-pasting:
glsl es 1.0 is VERY strict, so you should always check the code for possible errors beforehand, otherwise Chrome will make you go nuts. For example if you define a float variable like this:
float myFloat = 1;
you will get an error because 1 is not a float. Write 1.0 instead.
another popular one:
gl_FragColor is a vec4(). So if you pass a vec3 to it (just rgb for example), you'll get an error.
gl_FragColor = vec3(1.0, 1.0, 1.0); // <- wrong
gl_FragColor = vec4(1.0, 1.0, 1.0, 0.5); // <- right
following code is possible, too:
vec3 RGBcolor = vec3(1.0, 1.0, 1.0);
gl_FragColor = vec4(RGBcolor, 0.5);