Grey logo animation (#109)

Co-authored-by: Dean Herbert <pe@ppy.sh>

osu.Game.Resources/Shaders/sh_LogoAnimation.fs

@@ -0,0 +1,14 @@
+#include "sh_Utils.h"
+
+varying mediump vec2 v_TexCoord;
+varying lowp vec4 v_Colour;
+
+uniform lowp sampler2D m_Sampler;
+uniform lowp float progress;
+
+void main(void) 
+{
+    vec4 colour = toSRGB(texture2D(m_Sampler, v_TexCoord, -0.9));
+    
+    gl_FragColor = colour.r < progress ? toSRGB(vec4(v_Colour.rgb, v_Colour.a * colour.a)) : vec4(0);
+}