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.go

@@ -5,6 +5,7 @@
 package headless
 
 import (
+	"errors"
 	"image"
 	"image/color"
 	"runtime"
@@ -30,6 +31,33 @@ type context interface {
 	Release()
 }
 
+var (
+	newContextPrimary  func() (context, error)
+	newContextFallback func() (context, error)
+)
+
+func newContext() (context, error) {
+	funcs := []func() (context, error){newContextPrimary, newContextFallback}
+	var firstErr error
+	for _, f := range funcs {
+		if f == nil {
+			continue
+		}
+		c, err := f()
+		if err != nil {
+			if firstErr == nil {
+				firstErr = err
+			}
+			continue
+		}
+		return c, nil
+	}
+	if firstErr != nil {
+		return nil, firstErr
+	}
+	return nil, errors.New("x11: no available GPU backends")
+}
+
 // NewWindow creates a new headless window.
 func NewWindow(width, height int) (*Window, error) {
 	ctx, err := newContext()