Desktop: Render overlays with vello (#2965)

* Render overlays with vello

* Fix nix flake comments

* Rendering refactor with better names and code location

* Remove unnecessary overlay renders

* Post rebase fix

desktop/src/render/composite_shader.wgsl

@@ -0,0 +1,69 @@
+struct VertexOutput {
+	@builtin(position) clip_position: vec4<f32>,
+	@location(0) tex_coords: vec2<f32>,
+}
+
+@vertex
+fn vs_main(@builtin(vertex_index) vertex_index: u32) -> VertexOutput {
+	var out: VertexOutput;
+
+	let pos = array(
+		// 1st triangle
+		vec2f( -1.0, -1.0), // center
+		vec2f( 1.0, -1.0), // right, center
+		vec2f( -1.0, 1.0), // center, top
+
+		// 2nd triangle
+		vec2f( -1.0, 1.0), // center, top
+		vec2f( 1.0, -1.0), // right, center
+		vec2f( 1.0, 1.0), // right, top
+	);
+	let xy = pos[vertex_index];
+	out.clip_position = vec4f(xy , 0.0, 1.0);
+	let coords = (xy / 2. + 0.5);
+	out.tex_coords = vec2f(coords.x, 1. - coords.y);
+	return out;
+}
+
+
+struct Constants {
+	viewport_scale: vec2<f32>,
+	viewport_offset: vec2<f32>,
+};
+
+var<push_constant> constants: Constants;
+
+@group(0) @binding(0)
+var t_viewport: texture_2d<f32>;
+@group(0) @binding(1)
+var t_overlays: texture_2d<f32>;
+@group(0) @binding(2)
+var t_ui: texture_2d<f32>;
+@group(0) @binding(3)
+var s_diffuse: sampler;
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+	let ui = textureSample(t_ui, s_diffuse, in.tex_coords);
+	if (ui.a >= 0.999) {
+		return ui;
+	}
+
+	let vp_cord = (in.tex_coords - constants.viewport_offset) * constants.viewport_scale;
+
+	let ov = textureSample(t_overlays, s_diffuse, vp_cord);
+	let vp = textureSample(t_viewport, s_diffuse, vp_cord);
+
+	if (ov.a < 0.001) {
+		return blend(ui, vp);
+	}
+
+	let comp = blend(ov, vp);
+	return blend(ui, comp);
+}
+
+fn blend(fg: vec4<f32>, bg: vec4<f32>) -> vec4<f32> {
+	let a = fg.a + bg.a * (1.0 - fg.a);
+	let rgb = fg.rgb * fg.a + bg.rgb * bg.a * (1.0 - fg.a);
+	return vec4<f32>(rgb, a);
+}