app/internal/window,app/internal/gl: move gl.Context to package window

Package gl now only defines opengl types and functions.

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

app/internal/gl/gl.go

@@ -7,15 +7,6 @@ type (
 	Enum   uint
 )
 
-type Context interface {
-	Functions() *Functions
-	Present() error
-	MakeCurrent() error
-	Release()
-	Lock()
-	Unlock()
-}
-
 const (
 	ARRAY_BUFFER                          = 0x8892
 	BLEND                                 = 0xbe2