Added backdrop effects pipeline (blur)

draw/shaders/source/backdrop_blur.vert

@@ -0,0 +1,109 @@
+#version 450 core
+
+// Unified backdrop blur vertex shader.
+// Handles both H-blur (fullscreen triangle, mode 0) and V-blur+composite (instanced
+// unit-quad over Backdrop_Primitive storage buffer, mode 1) for the second PSO of
+// the backdrop bracket. The first PSO (downsample) uses backdrop_fullscreen.vert.
+//
+// No vertex buffer for either mode. Mode 0 uses gl_VertexIndex 0..2 for a single
+// fullscreen triangle; mode 1 uses gl_VertexIndex 0..5 for a unit-quad (two
+// triangles, TRIANGLELIST topology) and gl_InstanceIndex to select the primitive.
+//
+// Mode 0 viewport+scissor are CPU-set per layer-bracket to the work region (union
+// AABB of backdrop primitives + 3*max_sigma, clamped to swapchain bounds). Mode 1
+// renders into the main render target with the screen-space orthographic projection;
+// the per-primitive bounds drive the quad in screen space.
+//
+// Backdrop primitives have NO rotation — backdrop sampling is in screen space, so
+// a rotated mask over a stationary blur sample would look wrong.
+
+// --- Outputs to fragment shader ---
+// p_local: shape-local position in physical pixels (origin at shape center).
+//          Only meaningful in mode 1 (V-composite). Zero-init for mode 0.
+layout(location = 0) out vec2 p_local;
+// f_color: tint, unpacked from primitive.color. Only meaningful in mode 1.
+layout(location = 1) out mediump vec4 f_color;
+// f_half_size: RRect half extents in physical pixels (mode 1 only).
+layout(location = 2) flat out vec2 f_half_size;
+// f_radii: per-corner radii in physical pixels (mode 1 only).
+layout(location = 3) flat out vec4 f_radii;
+// f_half_feather: SDF anti-aliasing feather (mode 1 only).
+layout(location = 4) flat out float f_half_feather;
+
+// --- Uniforms (set 1) ---
+// Backdrop pipeline's own uniform block — distinct from the main pipeline's
+// Vertex_Uniforms. `mode` selects between H-blur (0) and V-composite (1).
+layout(set = 1, binding = 0) uniform Uniforms {
+    mat4 projection;
+    float dpi_scale;
+    uint mode; // 0 = H-blur, 1 = V-composite
+    vec2 _pad0;
+};
+
+// --- Backdrop primitive storage buffer (set 0) ---
+// 48 bytes, std430-natural layout (no implicit padding). vec4 members are
+// front-loaded so their 16-byte alignment is satisfied without holes; the
+// vec2 and scalar tail packs tight to land the struct at a clean 48-byte
+// stride (a multiple of 16, so the array stride needs no rounding either).
+// Field semantics match the CPU-side Backdrop_Primitive declared in
+// levlib/draw/pipeline_2d_backdrop.odin; keep both in sync.
+//
+// Backdrop primitives are tint-only in v1: outline is intentionally absent.
+// Future specialized effects (e.g. liquid-glass-style edges) would be a
+// dedicated primitive type with its own pipeline rather than a flag bit here.
+struct Backdrop_Primitive {
+    vec4 bounds; //  0-15: min_xy, max_xy (world-space)
+    vec4 radii; // 16-31: per-corner radii (physical px)
+    vec2 half_size; // 32-39: RRect half extents (physical px)
+    float half_feather; // 40-43: SDF anti-aliasing feather (physical px)
+    uint color; // 44-47: tint, packed RGBA u8x4
+};
+
+layout(std430, set = 0, binding = 0) readonly buffer Backdrop_Primitives {
+    Backdrop_Primitive primitives[];
+};
+
+void main() {
+    if (mode == 0u) {
+        // ---- Mode 0: H-blur fullscreen triangle ----
+        // 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);
+
+        // Mode 0 doesn't read the per-primitive varyings; zero-init for safety.
+        p_local = vec2(0.0);
+        f_color = vec4(0.0);
+        f_half_size = vec2(0.0);
+        f_radii = vec4(0.0);
+        f_half_feather = 0.0;
+    } else {
+        // ---- Mode 1: V-composite instanced unit-quad over Backdrop_Primitive ----
+        Backdrop_Primitive p = primitives[gl_InstanceIndex];
+
+        // Unit-quad corners for TRIANGLELIST (2 triangles, 6 vertices):
+        //   index 0 -> (0,0)   index 3 -> (0,1)
+        //   index 1 -> (1,0)   index 4 -> (1,0)
+        //   index 2 -> (0,1)   index 5 -> (1,1)
+        vec2 quad_corners[6] = vec2[6](
+                vec2(0.0, 0.0), vec2(1.0, 0.0), vec2(0.0, 1.0),
+                vec2(0.0, 1.0), vec2(1.0, 0.0), vec2(1.0, 1.0));
+        vec2 corner = quad_corners[gl_VertexIndex];
+
+        vec2 world_pos = mix(p.bounds.xy, p.bounds.zw, corner);
+        vec2 center = 0.5 * (p.bounds.xy + p.bounds.zw);
+
+        // Shape-local position in physical pixels (no rotation for backdrops).
+        p_local = (world_pos - center) * dpi_scale;
+
+        f_color = unpackUnorm4x8(p.color);
+        f_half_size = p.half_size;
+        f_radii = p.radii;
+        f_half_feather = p.half_feather;
+
+        gl_Position = projection * vec4(world_pos * dpi_scale, 0.0, 1.0);
+    }
+}