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_linux.go

@@ -20,13 +20,6 @@ type (
 	_EGLSurface = C.EGLSurface
 )
 
-var (
-	nilEGLSurface          _EGLSurface
-	nilEGLContext          _EGLContext
-	nilEGLConfig           _EGLConfig
-	nilEGLNativeWindowType _EGLNativeWindowType
-)
-
 func eglChooseConfig(disp _EGLDisplay, attribs []_EGLint) (_EGLConfig, bool) {
 	var cfg C.EGLConfig
 	var ncfg C.EGLint