put shell in subdir, and add nix package

shell/shaders/mask.frag

@@ -0,0 +1,25 @@
+#version 440
+
+layout(location = 0) in vec2 qt_TexCoord0;
+layout(location = 0) out vec4 fragColor;
+
+layout(binding = 1) uniform sampler2D source;
+layout(binding = 2) uniform sampler2D mask;
+
+layout(std140, binding = 0) uniform buf {
+    mat4 qt_Matrix;
+    float qt_Opacity;
+};
+
+void main() {
+    vec4 sourceColor = texture(source, qt_TexCoord0);
+    vec4 maskColor = texture(mask, qt_TexCoord0);
+
+    // Use the mask's luminance to determine opacity
+    float maskValue = dot(maskColor.rgb, vec3(0.299, 0.587, 0.114));
+
+    // Black areas of mask = transparent, white areas = opaque
+    sourceColor.a *= (1.0 - maskValue) * qt_Opacity;
+
+    fragColor = sourceColor;
+}