ui/sdl2: fix surface_gl_update_texture: Assertion 'gls' failed

ui/sdl2: fix surface_gl_update_texture: Assertion 'gls' failed

Before sdl2_gl_update() is called, sdl2_gl_switch() may decide to
destroy the console window and its associated shaders.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1644
Fixes: c84ab0a500a8 ("ui/console: optionally update after gfx switch")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Message-Id: <20230511074217.4171842-1-marcandre.lureau@redhat.com>
(cherry picked from commit b3a654d82ecf276b59a67b2fd688e11a0d8a0064)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>