http://jsdo.it/xl1/rD74 のシェーダ

drag.fs

precision mediump float;
uniform vec2 u_textureSize;

void main(){
  float x = u_textureSize.x - 50.0;
  gl_FragColor = vec4(x / 250.0, 0.0, 0.0, 1.0);
}

normal.vs

attribute vec4 a_position;
uniform mat4 u_projectionMatrix;

void main(){
  gl_Position = u_projectionMatrix * a_position;
}