[Ohrrpgce] SVN: teeemcee/11683 Fix crash when switching back to gfx_sdl after switching away from it wi

[subversion at HamsterRepublic.com]

teeemcee
2020-04-04 23:00:58 -0700 (Sat, 04 Apr 2020)
247
Fix crash when switching back to gfx_sdl after switching away from it with Ctrl-F7

I don't know why it's necessary, but this extra check fixes it and now
switching backend seems

Removed the "EXPERIMENTAL!" warning from the backend switching menu
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U   wip/backends.bas
U   wip/gfx_sdl.bas
U   wip/gfx_sdl2.bas
U   wip/whatsnew.txt