add flashlight shader

osu.Game.Resources/Shaders/sh_Flashlight.fs

@@ -0,0 +1,21 @@
+#ifdef GL_ES
+    precision mediump float;
+#endif
+
+varying vec2 v_Position;
+
+uniform vec2 mousePos;
+uniform float flashlightSize;
+
+const float smoothness = 0.1;
+
+void main(void)
+{
+    vec2 diff = mousePos - v_Position;
+
+    float dist = sqrt(diff.x * diff.x + diff.y * diff.y);
+
+    float smoothnessRange = flashlightSize * smoothness;
+
+    gl_FragColor = vec4(0.0, 0.0, 0.0, smoothstep(flashlightSize, flashlightSize + smoothnessRange, dist));
+}