[Ohrrpgce] SVN: teeemcee/10821 Re-do fix for gfx_sdl (now gfx_sdl2 too) keys getting stuck after backen

[subversion at HamsterRepublic.com]

teeemcee
2018-10-28 20:36:42 -0700 (Sun, 28 Oct 2018)
401
Re-do fix for gfx_sdl (now gfx_sdl2 too) keys getting stuck after backend reinit

I fixed this problem recently in r10452, but it turns out that fix was bad, it
only worked because gfx_backend_menu() calls multichoice() which calls
push_and_reset_gfxio_state() which causes gfx_sdl to recreate the window,
causing keys to be down.
Fixed it in a simpler way.

Also moved gfx_close to gfx_sdl_common.bas
---
U   wip/gfx_sdl.bas
U   wip/gfx_sdl2.bas
U   wip/gfx_sdl_common.bas
U   wip/gfx_sdl_common.bi