Use high precision for all shaders

medium precision causes visible seams between tiles on some drivers.

shader/atmos.frag

 #version 100
-precision mediump float;
+precision highp float;
 
 varying vec2 v_pos;
 

shader/atmos.vert

 #version 100
-precision mediump float;
+precision highp float;
 
 attribute vec2 position;
 

shader/map.frag

 #version 100
-precision mediump float;
+precision highp float;
 
 varying vec2 v_tex;
 varying vec4 v_tex_minmax;

shader/map.vert

 #version 100
-precision mediump float;
+precision highp float;
 
 attribute vec2 position;
 attribute vec2 tex_coord;

shader/map_tile_border.frag

 #version 100
-precision mediump float;
+precision highp float;
 
 varying vec2 v_tex;
 varying vec4 v_tex_minmax;

shader/marker.frag

 #version 100
-precision mediump float;
+precision highp float;
 
 varying vec2 v_tex;
 uniform sampler2D tex;

shader/marker.vert

 #version 100
-precision mediump float;
+precision highp float;
 
 attribute vec2 position;
 attribute vec2 tex_coord;

shader/ortho_tile.frag

 #version 100
-precision mediump float;
+precision highp float;
 
 varying vec2 v_tex;
 varying vec4 v_tex_minmax;

shader/ortho_tile.vert

 #version 100
-precision mediump float;
+precision highp float;
 
 attribute vec3 position;
 attribute vec2 tex_coord;