make lava slowly move over time

assets/fs/shaders/lava.f.glsl

@@ -1,9 +1,11 @@
 
 uniform sampler2D tex;
+uniform float shift;
 
 varying vec2 tex_coord_i;
 
 void main(void)
 {
-    gl_FragColor = texture2D(tex, tex_coord_i);
+    vec2 tex_coord = vec2(tex_coord_i.s + shift, tex_coord_i.t);
+    gl_FragColor = texture2D(tex, tex_coord);
 }