Backdrop Path + Cybersteel (#23)

Co-authored-by: Zachary Levy <zachary@sunforge.is>
Reviewed-on: #23

draw/shaders/source/backdrop_fullscreen.vert

@@ -0,0 +1,21 @@
+#version 450 core
+
+// Fullscreen-triangle vertex shader for the backdrop downsample and H-blur sub-passes.
+// Emits a single triangle covering NDC [-1,1]^2; the rasterizer clips edges outside.
+// No vertex buffer; uses gl_VertexIndex to pick corners.
+//
+// The CPU sets the viewport (and matching scissor) per layer-bracket to limit work to
+// the union AABB of the layer's backdrop primitives, expanded by 3*max_sigma and
+// clamped to swapchain bounds. The fragment shader uses gl_FragCoord (absolute pixel
+// space in the bound target) plus an inv-size uniform to compute its own UVs — see
+// each fragment shader for the per-pass sampling math.
+
+void main() {
+    // gl_VertexIndex 0 -> ( -1, -1)
+    // gl_VertexIndex 1 -> (  3, -1)
+    // gl_VertexIndex 2 -> ( -1,  3)
+    vec2 ndc = vec2(
+            (gl_VertexIndex == 1) ? 3.0 : -1.0,
+            (gl_VertexIndex == 2) ? 3.0 : -1.0);
+    gl_Position = vec4(ndc, 0.0, 1.0);
+}