gpu,app/internal/wm: add Metal port

The OpenGL (ES) implementations on Apple platforms are deprecated and
don't support GPU compute programs. This change adds support for the
replacement, the Metal GPU API.

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

app/internal/wm/gl_macos.m

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-// +build darwin,!ios
+// +build darwin,!ios,nometal
 
 @import AppKit;
 
@@ -8,6 +8,12 @@
 #include <OpenGL/OpenGL.h>
 #include "_cgo_export.h"
 
+CALayer *gio_layerFactory(void) {
+	@autoreleasepool {
+		return [CALayer layer];
+	}
+}
+
 CFTypeRef gio_createGLContext(void) {
 	@autoreleasepool {
 		NSOpenGLPixelFormatAttribute attr[] = {