use high on painting shaders

data/shaders/texture-alpha.glsl

@@ -15,15 +15,15 @@ void main()
 [[fragment]]
 
 uniform sampler2D tex;
-uniform mediump float alpha;
+uniform highp float alpha;
 uniform bool highlight;
 
-in mediump vec2 uv;
-out mediump vec4 frag;
+in highp vec2 uv;
+out highp vec4 frag;
 
 void main()
 {
-    mediump vec4 c = texture(tex, uv);
+    highp vec4 c = texture(tex, uv);
     frag = highlight ? 
         vec4(clamp(vec3(0.3) + c.rgb, vec3(0.0), vec3(1.0)), c.a) : 
         c * vec4(1.0, 1.0, 1.0, alpha);