subsurfaces: fix rendering problem with cairo-glesv2

when using cairo-glesv2 subsurface rendering code
implicitly calls eglMakeCurrent(dpy, NULL, NULL, ctx)
(since EGL_KHR_surfaceless_context is used), thus,
triangle_frame_callback:eglSwapBuffers returns EGL_BAD_SURFACE
error for all invocations other that the first one
1 file changed