move brush opacity from comp to stroke

data/shaders/comp-erase.glsl

@@ -20,7 +20,6 @@ uniform sampler2D tex;
 uniform sampler2D tex_stroke;
 uniform sampler2D tex_mask;
 uniform mediump float alpha;
-uniform mediump float stroke_alpha;
 uniform mediump vec2 resolution;
 uniform bool mask;
 
@@ -31,6 +30,6 @@ void main()
 {
     mediump vec4 base = texture(tex, uv);
     mediump vec4 stroke = texture(tex_stroke, uv);
-    stroke.a = mask ? stroke.a * stroke_alpha * blur(tex_mask, uv).r : stroke.a * stroke_alpha;
+    stroke.a = mask ? stroke.a * blur(tex_mask, uv).r : stroke.a;
     frag = vec4(base.rgb, clamp((base.a - stroke.a) * alpha, 0.0, 1.0));
 }