app/headless: remove OpenGL assumptions

To prepare package headless for multiple backends, refactor the common headless
driver to no longer assume an OpenGL context. Instead, introduce a headless
backend type and the OpenGL implementation, glBackend.

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

app/headless/headless_darwin.go

@@ -22,7 +22,7 @@ type nsContext struct {
 	prepared bool
 }
 
-func newContext() (context, error) {
+func newGLContext() (glContext, error) {
 	ctx := C.gio_headless_newContext()
 	return &nsContext{ctx: ctx, c: new(glimpl.Functions)}, nil
 }