Fix #135659: Some types of motion are incorrect at low step counts with MetalRT

Following #136253, this PR enables decomposed MetalRT motion
interpolation on macOS 15.6. The bounding box issue is fixed
in the latest macOS 15.6 beta (24G5054d).

Pull Request: https://projects.blender.org/blender/blender/pulls/141207

intern/cycles/device/metal/device_impl.mm

@@ -105,7 +105,8 @@ MetalDevice::MetalDevice(const DeviceInfo &info, Stats &stats, Profiler &profile
     /* Use "Ray tracing with per component motion interpolation" if available.
      * Requires Apple9 support (https://developer.apple.com/metal/Metal-Feature-Set-Tables.pdf). */
     if (use_metalrt && [mtlDevice supportsFamily:MTLGPUFamilyApple9]) {
-      if (@available(macos 15.0, *)) {
+      /* Concave motion paths weren't correctly bounded prior to macOS 15.6 (#136253). */
+      if (@available(macos 15.6, *)) {
         use_pcmi = DebugFlags().metal.use_metalrt_pcmi;
       }
     }

intern/cycles/util/debug.h

@@ -102,9 +102,8 @@ class DebugFlags {
     bool use_async_pso_creation = true;
 
     /* Whether to use per-component motion interpolation.
-     * TODO: Enable by default when "multi step velocity motion blur" fail is fixed.
      */
-    bool use_metalrt_pcmi = false;
+    bool use_metalrt_pcmi = true;
   };
 
   /* Get instance of debug flags registry. */