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>

app/os_wayland.go

@@ -220,6 +220,11 @@ var callbackMap sync.Map
 // order of preference.
 var clipboardMimeTypes = []string{"text/plain;charset=utf8", "UTF8_STRING", "text/plain", "TEXT", "STRING"}
 
+var (
+	newWaylandEGLContext    func(w *window) (context, error)
+	newWaylandVulkanContext func(w *window) (context, error)
+)
+
 func init() {
 	wlDriver = newWLWindow
 }
@@ -1476,6 +1481,28 @@ func (w *window) SetInputHint(_ key.InputHint) {}
 // Close the window. Not implemented for Wayland.
 func (w *window) Close() {}
 
+func (w *window) NewContext() (context, error) {
+	var firstErr error
+	if f := newWaylandVulkanContext; f != nil {
+		c, err := f(w)
+		if err == nil {
+			return c, nil
+		}
+		firstErr = err
+	}
+	if f := newWaylandEGLContext; f != nil {
+		c, err := f(w)
+		if err == nil {
+			return c, nil
+		}
+		firstErr = err
+	}
+	if firstErr != nil {
+		return nil, firstErr
+	}
+	return nil, errors.New("wayland: no available GPU backends")
+}
+
 // detectUIScale reports the system UI scale, or 1.0 if it fails.
 func detectUIScale() float32 {
 	// TODO: What about other window environments?