[Ohrrpgce] SVN: teeemcee/12428 Fix uninitialised gfx_init, gfx_set_window_size ptrs when switching gfx

[subversion at HamsterRepublic.com]

teeemcee
2021-09-12 04:45:25 -0700 (Sun, 12 Sep 2021)
385
Fix uninitialised gfx_init, gfx_set_window_size ptrs when switching gfx backends

If you switched to gfx_directx from any other backend but its gfx_Initialize
failed, gfx_init for the previous backend got called, and no working backend
was loaded!

If you switched from gfx_sdl2 to any other backend, gfx_sdl2_set_window_size
would still get called!

(Copied from r12421 on wip branch)
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U   rel/hrodvitnir/backends.bas