egl/wgl: Fixes warnings when compile with vs2019

```
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_add_config':
../../src/egl/drivers/wgl/egl_wgl.c:67:17: warning: variable 'wgl_sizes' set but not used
[-Wunused-but-set-variable]
   67 |    unsigned int wgl_sizes[4] = { 0, 0, 0, 0 };
      |                 ^~~~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c:66:8: warning: variable 'wgl_shifts' set but not used
[-Wunused-but-set-variable]
   66 |    int wgl_shifts[4] = { -1, -1, -1, -1 };
      |        ^~~~~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c:63:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
   63 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_destroy_context':
../../src/egl/drivers/wgl/egl_wgl.c:517:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
  517 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_make_current':
../../src/egl/drivers/wgl/egl_wgl.c:575:28: warning: variable 'old_wgl_dpy' set but not used [-Wunused-but-set-variable]
  575 |    struct wgl_egl_display *old_wgl_dpy = NULL;
      |                            ^~~~~~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_query_surface':
../../src/egl/drivers/wgl/egl_wgl.c:772:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
  772 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_bind_tex_image':
../../src/egl/drivers/wgl/egl_wgl.c:793:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
  793 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_swap_buffers':
../../src/egl/drivers/wgl/egl_wgl.c:857:28: warning: unused variable 'wgl_disp' [-Wunused-variable]
  857 |    struct wgl_egl_display *wgl_disp = wgl_egl_display(disp);
      |                            ^~~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_create_image_khr_texture':
../../src/egl/drivers/wgl/egl_wgl.c:911:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
  911 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_create_image_khr_renderbuffer':
../../src/egl/drivers/wgl/egl_wgl.c:980:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
  980 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_create_sync_khr':
../../src/egl/drivers/wgl/egl_wgl.c:1047:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
 1047 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
../../src/egl/drivers/wgl/egl_wgl.c: In function 'wgl_wait_sync_khr':
../../src/egl/drivers/wgl/egl_wgl.c:1163:28: warning: unused variable 'wgl_dpy' [-Wunused-variable]
 1163 |    struct wgl_egl_display *wgl_dpy = wgl_egl_display(disp);
      |                            ^~~~~~~
[3/3] Linking target src/egl/libEGL.dll
```

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>