fix for highp vs mediump

Author: monfera

lib/shaders/line-fragment.glsl

@@ -1,4 +1,4 @@
-precision mediump float;
+precision highp float;
 
 uniform vec4 color;
 uniform vec2 screenShape;

lib/shaders/line-vertex.glsl

@@ -1,4 +1,4 @@
-precision mediump float;
+precision highp float;
 
 attribute vec2 aHi, aLo, dHi, dLo;