gpu: Improve conic gradient rendering

1. Compute the fwidth() twice with offset offsets
   That way, we avoid glitches at the boundary between 0.0 and 1.0,
   because by offsetting it by 0.5, that boundary goes away.
   Then we take the min() of both which gives us the one we care about.

2. Set the gradient to repeating
   By doing that, we don't get values at the 0.0/1.0 boundary clamped,
   but things smoothly transition.
   This smoothes the line at that boundary and makes it look just like
   every other line.

gsk/gpu/shaders/pattern.glsl

@@ -187,10 +187,12 @@ conic_gradient_pattern (inout uint reader,
   /* scaling modifies angles, so be sure to use right coordinate system */
   pos = pos / push.scale - center;
   float offset = atan (pos.y, pos.x);
-  offset = fract (degrees (offset + angle) / 360.0);
-  float d_offset = 0.5 * fwidth (offset);
+  offset = degrees (offset + angle) / 360.0;
+  float overflow = fract (offset + 0.5);
+  offset = fract (offset);
+  float d_offset = max (0.00001, 0.5 * min (fwidth (offset), fwidth (overflow)));
 
-  return gradient_get_color (gradient, offset - d_offset, offset + d_offset);
+  return gradient_get_color_repeating (gradient, offset - d_offset, offset + d_offset);
 }
 
 vec4