Bring CursorTrail shader up to speed with framework.

osu.Game.Resources/Shaders/sh_CursorTrail.vs

@@ -1,16 +1,29 @@
 attribute vec2 m_Position;
+attribute vec4 m_Colour;
 attribute vec2 m_TexCoord;
-attribute float m_Time;
 
+varying vec2 v_DrawingPosition;
+varying vec2 v_MaskingPosition;
 varying vec4 v_Colour;
 varying vec2 v_TexCoord;
 
 uniform mat4 g_ProjMatrix;
+uniform mat3 g_ToMaskingSpace;
+uniform mat3 g_ToDrawingSpace;
+
 uniform float g_FadeClock;
 
 void main(void)
 {
-	gl_Position = g_ProjMatrix * vec4(m_Position, 1.0, 1.0);
-	v_Colour = vec4(1.0, 1.0, 1.0, m_Time - g_FadeClock);
+	// Transform to position to masking space.
+	vec3 maskingPos = g_ToMaskingSpace * vec3(m_Position, 1.0);
+	v_MaskingPosition = maskingPos.xy / maskingPos.z;
+
+	// Transform to position to masking space.
+	vec3 drawingPos = g_ToDrawingSpace * vec3(m_Position, 1.0);
+	v_DrawingPosition = drawingPos.xy / drawingPos.z;
+
+	v_Colour = vec4(m_Colour.rgb, clamp(m_Colour.a - g_FadeClock, 0.0, 1.0));
 	v_TexCoord = m_TexCoord;
+	gl_Position = g_ProjMatrix * vec4(m_Position, 1.0, 1.0);
 }