Only mark diff parameter as high precision

osu.Game.Resources/Shaders/sh_CircularFlashlight.fs

@@ -1,6 +1,7 @@
 #include "sh_Flashlight.h"
 
-vec4 getColourAt(vec2 diff, vec2 size, vec4 originalColour)
+// highp diff prevents loss in precision when calculating smoothing distance on GL_ES platforms
+vec4 getColourAt(highp vec2 diff, vec2 size, vec4 originalColour)
 {
     float dist = length(diff);
     float flashlightRadius = length(size);

osu.Game.Resources/Shaders/sh_Flashlight.h

@@ -1,5 +1,5 @@
 #ifdef GL_ES
-    precision highp float;
+    precision mediump float;
 #endif
 
 varying vec2 v_Position;