gpu/internal/opengl: rename and make internal the OpenGL driver

It is no longer necessary for outside users of package gpu to explicitly
initialize a specific driver. The Direct3D driver is already internal,
this moves the OpenGL driver internally as well. The rename to opengl is
to avoid the name clash with the low-level "gioui.org/internal/glimpl"
package that we're about to rename.

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

gpu/gpu.go

@@ -30,8 +30,8 @@ import (
 	"gioui.org/op/clip"
 
 	// Register backends.
-	_ "gioui.org/gpu/gl"
 	_ "gioui.org/gpu/internal/d3d11"
+	_ "gioui.org/gpu/internal/opengl"
 )
 
 type GPU interface {