threaded GL, fails at restoring EGL context

#include <SDL.h>
#include <GLES2/gl2.h>

#include <stdbool.h>

#ifdef __ANDROID__
#  include <android/log.h>
#  define PRINT(s) __android_log_write(ANDROID_LOG_DEBUG, "glthreaded", s)
#else
#  define PRINT(s) fprintf(stderr, s "\n")
#endif

struct ThreadData
{
    SDL_Window *win;
    int winW, winH;
    SDL_mutex *mut;
    bool foreground;
    bool running;
};

void *threadFunction(struct ThreadData *d)
{
    SDL_GLContext glCtx = SDL_GL_CreateContext(d->win);

    glClearColor(0, 1.0, 1.0, 1.0);

    while (d->running)
    {
        SDL_LockMutex(d->mut);
        glViewport(0, 0, d->winW, d->winH);
        glClear(GL_COLOR_BUFFER_BIT);
        SDL_GL_SwapWindow(d->win);
        SDL_UnlockMutex(d->mut);

        SDL_Delay(16);
    }

    SDL_GL_DeleteContext(glCtx);
}

int eventFilter(struct ThreadData *d, SDL_Event *e)
{
    switch (e->type)
    {
    case SDL_APP_WILLENTERBACKGROUND :
        if (d->foreground)
        {
            PRINT("About to enter background, sleeping render thread");
            d->foreground = false;
            SDL_LockMutex(d->mut);
        }

        return 0;

    case SDL_APP_DIDENTERFOREGROUND :
        if (!d->foreground)
        {
            PRINT("Did enter foreground,  waking up render thread");
            d->foreground = true;
            SDL_UnlockMutex(d->mut);
        }

        return 0;

    default:
        return 1;
    }
}

int main(int argc, char *argv[])
{
    struct ThreadData d;

    SDL_Init(SDL_INIT_VIDEO);

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);

    d.win = SDL_CreateWindow("threaded gl",
                             SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
                             100, 100, SDL_WINDOW_RESIZABLE | SDL_WINDOW_OPENGL);
    SDL_GetWindowSize(d.win, &d.winW, &d.winH);

    d.mut = SDL_CreateMutex();
    d.foreground = true;
    d.running = true;

    SDL_SetEventFilter((SDL_EventFilter) eventFilter, &d);

    SDL_Thread *thread = SDL_CreateThread((SDL_ThreadFunction) threadFunction, "gl", &d);

    while (d.running)
    {
        SDL_Event e;
        while (SDL_PollEvent(&e) > 0)
        {
            switch (e.type)
            {
            case SDL_QUIT :
                d.running = false;
                break;

            case SDL_WINDOWEVENT :
                if (e.window.event == SDL_WINDOWEVENT_SIZE_CHANGED)
                {
                    d.winW = e.window.data1;
                    d.winH = e.window.data2;
                    PRINT("Window size changed");
                    break;
                }

            /* Just for testing on the desktop */
            case SDL_KEYDOWN :
                if (e.key.keysym.scancode == SDL_SCANCODE_F1)
                {
                    e.type = SDL_APP_WILLENTERBACKGROUND;
                    SDL_PushEvent(&e);
                    break;
                }

            case SDL_KEYUP :
                if (e.key.keysym.scancode == SDL_SCANCODE_F1)
                {
                    e.type = SDL_APP_DIDENTERFOREGROUND;
                    SDL_PushEvent(&e);
                    break;
                }

            default:
                break;
            }
        }
    }

    SDL_WaitThread(thread, 0);

    SDL_DestroyMutex(d.mut);
    SDL_DestroyWindow(d.win);

    SDL_Quit();
}