ADD: aspect ratio preserving completed

shaders/present_letterbox.frag

@@ -0,0 +1,36 @@
+#version 450
+
+layout(location=0) in vec2 inUV;
+layout(location=0) out vec4 outColor;
+
+layout(set=0, binding=0) uniform sampler2D uSrc;
+
+layout(push_constant) uniform Push
+{
+    vec2 rect_min;   // normalized (0..1) min corner in swapchain UV space
+    vec2 rect_size;  // normalized size in swapchain UV space
+} pc;
+
+void main()
+{
+    if (pc.rect_size.x <= 0.0 || pc.rect_size.y <= 0.0)
+    {
+        outColor = vec4(0.0, 0.0, 0.0, 1.0);
+        return;
+    }
+
+    vec2 rect_max = pc.rect_min + pc.rect_size;
+    vec2 uv = inUV;
+
+    if (uv.x < pc.rect_min.x || uv.y < pc.rect_min.y || uv.x > rect_max.x || uv.y > rect_max.y)
+    {
+        outColor = vec4(0.0, 0.0, 0.0, 1.0);
+        return;
+    }
+
+    vec2 local = (uv - pc.rect_min) / pc.rect_size;
+    local = clamp(local, vec2(0.0), vec2(1.0));
+
+    outColor = texture(uSrc, local);
+}
+