tileMap.fsh

uniform vec2 worldSize;
uniform vec2 tileScale;
uniform sampler2D tileIds; //This is the map you update every frame
uniform sampler2D tileSheet;  //This has the texture data
varying vec3 worldCoord;

void main() {
  vec2 worldUV = worldCoord.xy / worldCoord.z;
  vec2 tileCoord = tileScale * (floor(255.0 * texture2D(tileIds, floor(worldUV) / worldSize).ra) + fract(worldUV));
  gl_FragColor = texture2D(tileSheet, tileCoord);
}

tileMap.vsh

attribute vec2 position;
uniform mat3 viewMatrix;
varying vec3 worldCoord;

void main() {
  vec3 worldCoord = viewMatrix * vec3(pixelCoordinate, 1.0);
  gl_Position = position;
}