Fix inset box shadows with blur

In commit d570574 ("Rework box shadows to support border radii
properly ..."), the conditional of the box shadow fragment shader that
handles inset shadows was lost. Restore it.

res/box_shadow.fs.glsl

@@ -141,6 +141,8 @@ void main(void) {
     vec2 radii = vBorderRadii.xy;
     float sigma = vBlurRadius / 2.0;
     float value = color(pos, p0Rect, p1Rect, radii, sigma);
-    SetFragColor(vec4(vColor.rgb, max(value, 0.0)));
+
+    value = max(value, 0.0);
+    SetFragColor(vec4(vColor.rgb, vColor.a == 0.0 ? 1.0 - value : value));
 }