Fixes bug where certain values of barrel distortion would cause alpha…

…=0 pixel in center of source. (#36)

Closes #23

data/shaders/crt-composite.effect

@@ -39,7 +39,9 @@ float4 mainImage(VertData v_in) : TARGET
 	// Anti-alias barrel distortion edges
 	float d = min(min(uv.x, 1. - uv.x), min(uv.y, 1. - uv.y));	
 	float w = length(float2(ddx(d), ddy(d)));
-	float aa_mask = smoothstep(-0.7, 0.7, (abs(frac(d - 0.25) - 0.5) - 0.25) / w);
+	// The check for d < 0.49 below likely can be fixed to something
+	// that doesn't branch.
+	float aa_mask = d < 0.49 ? smoothstep(-0.7, 0.7, (abs(frac(d - 0.25) - 0.5) - 0.25) / w) : 1.0;
 
 	float4 color = image.Sample(textureSampler, uv);
 	float4 blur_color = blur_image.Sample(textureSampler, uv);