[d3d9] Avoid depth degenerate viewports

src/d3d9/d3d9_device.cpp

@@ -5624,12 +5624,16 @@ namespace dxvk {
     // Originally we did this only for powers of two
     // resolutions but since NEAREST filtering fixed to
     // truncate, we need to do this all the time now.
-    float cf = 0.5f - (1.0f / 128.0f);
+    constexpr float cf = 0.5f - (1.0f / 128.0f);
+
+    // How much to bias MinZ by to avoid a depth
+    // degenerate viewport.
+    constexpr float zBias = 0.001f;
 
     viewport = VkViewport{
       float(vp.X)     + cf,    float(vp.Height + vp.Y) + cf,
       float(vp.Width),        -float(vp.Height),
-      vp.MinZ,                 vp.MaxZ,
+      vp.MinZ,                 std::max(vp.MaxZ, vp.MinZ + zBias),
     };
 
     // Scissor rectangles. Vulkan does not provide an easy way