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/api.go

@@ -8,7 +8,7 @@ import "gioui.org/gpu/internal/driver"
 // There is an API type for each supported GPU API such as OpenGL and Direct3D.
 type API = driver.API
 
-// A RenderTarget denotest the destination framebuffer for a frame.
+// A RenderTarget denotes the destination framebuffer for a frame.
 type RenderTarget = driver.RenderTarget
 
 // OpenGLRenderTarget is a render target suitable for the OpenGL backend.
@@ -20,6 +20,9 @@ type Direct3D11RenderTarget = driver.Direct3D11RenderTarget
 // MetalRenderTarget is a render target suitable for the Metal backend.
 type MetalRenderTarget = driver.MetalRenderTarget
 
+// VulkanRenderTarget is a render target suitable for the Vulkan backend.
+type VulkanRenderTarget = driver.VulkanRenderTarget
+
 // OpenGL denotes the OpenGL or OpenGL ES API.
 type OpenGL = driver.OpenGL
 
@@ -29,6 +32,9 @@ type Direct3D11 = driver.Direct3D11
 // Metal denotes the Apple Metal API.
 type Metal = driver.Metal
 
+// Vulkan denotes the Vulkan API.
+type Vulkan = driver.Vulkan
+
 // ErrDeviceLost is returned from GPU operations when the underlying GPU device
 // is lost and should be recreated.
 var ErrDeviceLost = driver.ErrDeviceLost