ui/app: make opengl objects structs

WebGL use opaque values for object handles, not integers. To ensure
portability, wrap handle types with a struct.

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

ui/app/egl_windows.go

@@ -41,13 +41,6 @@ var (
 	_eglQueryString         = libEGL.NewProc("eglQueryString")
 )
 
-const (
-	nilEGLSurface          _EGLSurface          = 0
-	nilEGLContext          _EGLContext          = 0
-	nilEGLConfig           _EGLConfig           = 0
-	nilEGLNativeWindowType _EGLNativeWindowType = 0
-)
-
 func init() {
 	mustLoadDLL(libEGL, "libEGL.dll")
 	mustLoadDLL(gl.LibGLESv2, "libGLESv2.dll")