mask: handle alpha channel too when inverting

app/node/distort/mask/mask.cpp

@@ -47,7 +47,7 @@ ShaderCode MaskDistortNode::GetShaderCode(const ShaderRequest &request) const
   } else if (request.id == QStringLiteral("feather")) {
     return ShaderCode(FileFunctions::ReadFileAsString(QStringLiteral(":/shaders/blur.frag")));
   } else if (request.id == QStringLiteral("invert")) {
-    return ShaderCode(FileFunctions::ReadFileAsString(QStringLiteral(":/shaders/invertrgb.frag")));
+    return ShaderCode(FileFunctions::ReadFileAsString(QStringLiteral(":/shaders/invertrgba.frag")));
   } else {
     return super::GetShaderCode(request);
   }

app/shaders/invertrgba.frag

@@ -0,0 +1,12 @@
+// Input texture
+uniform sampler2D tex_in;
+
+// Input texture coordinate
+in vec2 ove_texcoord;
+out vec4 frag_color;
+
+void main() {
+    vec4 color = texture(tex_in, ove_texcoord);
+    color = 1.0 - color;
+    frag_color = color;
+}