fix color bleeding

data/shaders/stroke-dilate.glsl

@@ -0,0 +1,42 @@
+[[vertex]]
+
+uniform mat4 mvp;
+
+in vec4 pos;
+in vec2 uvs;
+out vec2 uv;
+
+void main()
+{
+    gl_Position = mvp * vec4(pos.xy, 0.0, 1.0);
+    uv = uvs;
+}
+
+[[fragment]]
+
+uniform sampler2D tex_bg;
+
+in highp vec2 uv;
+out highp vec4 frag;
+
+void main()
+{
+    highp vec4 bg = texture(tex_bg, uv);
+    if (bg.a == 0.0)
+    {
+        highp vec4 sum = vec4(0.0);
+        for (int y = -1; y <= 1; y++)
+        {
+            for (int x = -1; x <= 1; x++)
+            {
+                highp vec4 c = textureOffset(tex_bg, uv, ivec2(x, y));
+                sum += vec4(c.rgb * c.a, c.a);
+            }
+        }
+        frag = sum.a > 0.0 ? vec4(sum.rgb / sum.a, 0.0) : bg;
+    }
+    else
+    {
+        frag = bg;
+    }
+}