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_egl.go

@@ -8,6 +8,6 @@ import (
 	"gioui.org/app/internal/egl"
 )
 
-func newContext() (context, error) {
+func newGLContext() (glContext, error) {
 	return egl.NewContext(egl.EGL_DEFAULT_DISPLAY)
 }