move brush opacity from comp to stroke

data/shaders/stroke.glsl

@@ -30,6 +30,7 @@ uniform mediump sampler2D tex_mix;
 uniform mediump vec4 col;
 uniform mediump vec2 resolution;
 uniform mediump float alpha;
+uniform mediump float opacity;
 uniform mediump float noise;
 uniform mediump float mix_alpha;
 uniform mediump float wet;
@@ -99,11 +100,11 @@ void main()
         fg.rgb = mix(fg.rgb, mbg.rgb, mix_alpha * mbg.a);
     }
 
-    mediump float contribution = (1.0 - bg.a) * fg.a;
+    mediump float contribution = (opacity - bg.a) * fg.a;
     mediump float alpha_tot = bg.a + contribution;
     mediump vec3 rgb = mix(bg.rgb, fg.rgb, fg.a / alpha_tot);
     mediump vec4 frag_wet = vec4(rgb, max(bg.a, fg.a * 1.2));
-    mediump vec4 frag_dry = vec4(rgb, alpha_tot);
+    mediump vec4 frag_dry = vec4(rgb, min(alpha_tot, opacity));
     frag = mix(frag_dry, frag_wet, wet);
 }