app,app/headless: expose Backends from window and headless contexts

This is a refactoring change to prepare for another gpu.Backend
implementations.

Notably, app/loop.go no longer imports gpu/gl.

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

app/headless/headless_js.go

@@ -7,6 +7,8 @@ import (
 	"syscall/js"
 
 	"gioui.org/app/internal/glimpl"
+	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
 )
 
 type jsContext struct {
@@ -37,6 +39,10 @@ func newContext() (*jsContext, error) {
 	return c, nil
 }
 
+func (c *jsContext) Backend() (gpu.Backend, error) {
+	return gl.NewBackend(c.f)
+}
+
 func (c *jsContext) Functions() *glimpl.Functions {
 	return c.f
 }