app,gpu,internal/vk: add Vulkan port for Wayland, X11, Android

This change implements a Vulkan port for the two renderers, old and
compute. Run with GIORENDERER=forcecompute to test the compute renderer.

To shake out bugs faster, it is also made the default on systems that
support it. To disable Vulkan and force the use of OpenGL, use the
`novulkan` tag:

$ go run -tags novulkan gioui.org/example/kitchen

Don't forget to file an issue describing the issue that prompted the use
of the tag.

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

gpu/headless/headless_darwin.go

@@ -37,17 +37,19 @@ type mtlContext struct {
 	queue C.CFTypeRef
 }
 
-func newContext() (context, error) {
-	dev := C.createDevice()
-	if dev == 0 {
-		return nil, errors.New("headless: failed to create Metal device")
+func init() {
+	newContextPrimary = func() (context, error) {
+		dev := C.createDevice()
+		if dev == 0 {
+			return nil, errors.New("headless: failed to create Metal device")
+		}
+		queue := C.newCommandQueue(dev)
+		if queue == 0 {
+			C.CFRelease(dev)
+			return nil, errors.New("headless: failed to create MTLQueue")
+		}
+		return &mtlContext{dev: dev, queue: queue}, nil
 	}
-	queue := C.newCommandQueue(dev)
-	if queue == 0 {
-		C.CFRelease(dev)
-		return nil, errors.New("headless: failed to create MTLQueue")
-	}
-	return &mtlContext{dev: dev, queue: queue}, nil
 }
 
 func (c *mtlContext) API() gpu.API {