add color burn, fragment early discard

data/shaders/stroke.glsl

@@ -59,6 +59,10 @@ void main()
     mediump float brush_alpha = ( 1.0 - texture(tex, uv/q).r ) * alpha;
     mediump vec4 fg = vec4(col.rgb, brush_alpha);
 
+    // early discard
+    if (fg.a == 0.0)
+        discard;
+
     if (use_pattern)
     {
         mediump vec2 rscale = resolution / vec2(512.0);
@@ -84,7 +88,7 @@ void main()
     fg.a *= 1.0-rand(uv2+uv)*noise;
 
     // no need to go further
-    if (fg.a == 0.0)
+    if (fg.a <= 0.0)
         discard;
 
     if (mix_alpha > 0.0)