gfx_sdl2 doesn't recenter when the window zoom is changed

[rversteegen]

kylekrack and Ravancloak complained that unlike gfx_sdl, gfx_sdl2 does not recenter the window when you change its zoom (from F9 or Cmd-1/2/3/4). Instead the top-left corner remains fixed.

This is because gfx_sdl2_recenter_window_hint called from gfx_sdl2_set_window_size does nothing, because the window isn't recreated, unlike gfx_sdl. Need to manually tell the window to recenter.

[rversteegen]

Fixed in r12419 (6fca195)