gpu,app: [Metal] don't store CFTypeRef values in pointer types

CFTypeRefs may not always contain valid pointers, so they must not be
stored in pointer types lest the Go runtime treats them as such.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/headless/headless_darwin.go

@@ -4,7 +4,6 @@ package headless
 
 import (
 	"errors"
-	"unsafe"
 
 	"gioui.org/gpu"
 	_ "gioui.org/internal/cocoainit"
@@ -54,8 +53,8 @@ func init() {
 
 func (c *mtlContext) API() gpu.API {
 	return gpu.Metal{
-		Device:      unsafe.Pointer(c.dev),
-		Queue:       unsafe.Pointer(c.queue),
+		Device:      uintptr(c.dev),
+		Queue:       uintptr(c.queue),
 		PixelFormat: int(C.MTLPixelFormatRGBA8Unorm_sRGB),
 	}
 }