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.go

@@ -26,6 +26,7 @@ type Window struct {
 
 type context interface {
 	Functions() *glimpl.Functions
+	Backend() (gpu.Backend, error)
 	MakeCurrent() error
 	ReleaseCurrent()
 	Release()
@@ -53,7 +54,7 @@ func NewWindow(width, height int) (*Window, error) {
 			ctx.Release()
 			return err
 		}
-		backend, err := gl.NewBackend(f)
+		backend, err := ctx.Backend()
 		if err != nil {
 			fbo.Release()
 			ctx.Release()

app/headless/headless_darwin.go

@@ -4,6 +4,8 @@ package headless
 
 import (
 	"gioui.org/app/internal/glimpl"
+	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
 )
 
 /*
@@ -38,6 +40,10 @@ func (c *nsContext) ReleaseCurrent() {
 	C.gio_headless_clearCurrentContext(c.ctx)
 }
 
+func (c *nsContext) Backend() (gpu.Backend, error) {
+	return gl.NewBackend(c.c)
+}
+
 func (c *nsContext) Functions() *glimpl.Functions {
 	return c.c
 }

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
 }