graphicsdriver/metal/mtl: Bug fix: Guard the handler map by a mutex

The handler is invoked in another thread than the main thread.

internal/graphicsdriver/metal/mtl/mtl.go

@@ -27,6 +27,7 @@ package mtl
 import (
 	"errors"
 	"fmt"
+	"sync"
 	"unsafe"
 )
 
@@ -568,20 +569,30 @@ func (cb CommandBuffer) WaitUntilCompleted() {
 	C.CommandBuffer_WaitUntilCompleted(cb.commandBuffer)
 }
 
-var commandBufferCompletedHandlers = map[unsafe.Pointer]func(){}
+var (
+	commandBufferCompletedHandlers  = map[unsafe.Pointer]func(){}
+	commandBufferCompletedHandlersM sync.Mutex
+)
 
 // AddCompletedHandler registers a block of code that Metal calls immediately after the GPU finishes executing the commands in the command buffer.
 //
 // Reference: https://developer.apple.com/documentation/metal/mtlcommandbuffer/1442997-addcompletedhandler
 func (cb CommandBuffer) AddCompletedHandler(f func()) {
+	commandBufferCompletedHandlersM.Lock()
 	commandBufferCompletedHandlers[cb.commandBuffer] = f
+	commandBufferCompletedHandlersM.Unlock()
+
 	C.CommandBuffer_AddCompletedHandler(cb.commandBuffer)
 }
 
 //export commandBufferCompletedCallback
 func commandBufferCompletedCallback(commandBuffer unsafe.Pointer) {
 	f := commandBufferCompletedHandlers[commandBuffer]
+
+	commandBufferCompletedHandlersM.Lock()
 	delete(commandBufferCompletedHandlers, commandBuffer)
+	commandBufferCompletedHandlersM.Unlock()
+
 	f()
 }