Fix: Metal parallel shader compilation does not respect -t option

This command line option to control the number of threads was already taken
into account for OpenGL and Vulkan.

source/blender/gpu/metal/mtl_backend.mm

@@ -8,6 +8,8 @@
 
 #include <cstring>
 
+#include "BLI_threads.h"
+
 #include "BKE_global.hh"
 
 #include "gpu_backend.hh"
@@ -523,8 +525,10 @@ void MTLBackend::capabilities_init(MTLContext *ctx)
 
   GCaps.geometry_shader_support = false;
 
-  /* Compile shaders on performance cores but leave one free so UI is still responsive */
-  GCaps.max_parallel_compilations = MTLBackend::capabilities.num_performance_cores - 1;
+  /* Compile shaders on performance cores but leave one free so UI is still responsive.
+   * Also respect command line option to reduce number of threads. */
+  GCaps.max_parallel_compilations = std::min(BLI_system_thread_count(),
+                                             MTLBackend::capabilities.num_performance_cores - 1);
 
   /* Maximum buffer bindings: 31. Consider required slot for uniforms/UBOs/Vertex attributes.
    * Can use argument buffers if a higher limit is required. */