gpu,gpu/gl: introduce Backend

A recent change made the OpenGL functions an interface of the functions
required for the implementation of GPU, a renderer for Gio operations.
That allowed for running Gio on external systems where OpenGL is
available.

However, to allow for non-OpenGL flavored backends such as Vulkan,
Metal and Direct3D, this change introduces Backend for the high-level
operations required by GPU. This change also adds a concrete backend
to package gl.

Type Backend is a first cut heavily based on OpenGL. Future changes will add
more backends, where the Backend interface quite possibly will need refinement.

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

app/headless/headless.go

@@ -51,7 +51,13 @@ func NewWindow(width, height int) (*Window, error) {
 			ctx.Release()
 			return err
 		}
-		gpu, err := gpu.New(f)
+		backend, err := gl.NewBackend(f)
+		if err != nil {
+			fbo.Release()
+			ctx.Release()
+			return err
+		}
+		gpu, err := gpu.New(backend)
 		if err != nil {
 			fbo.Release()
 			ctx.Release()