Crash when using display filters on some systems (fix included) about openxcom HOT 4 OPEN

Ok... I've just tested my 'fix' on Windows, and the results are not great. It's causing some crazy flickering weirdness. So I guess I haven't worked out a fix after all. Sorry. I'll let you know if I learn more.

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There's already a SDL_GL_SwapBuffers here: https://github.com/OpenXcom/OpenXcom/blob/master/src/Engine/Zoom.cpp#L680
Maybe it's in the wrong place?

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Right. The name flipWithZoom does imply that it should be flipping the display - but by the looks of it, it only flips in OpenGL mode.

If I simply comment out SDL_GL_SwapBuffers() line in Zoom::flipWithZoom, so that it is only called in my modified version of Screen::flip(), then the flicking problem in Windows is fixed. (It makes sense, because if we flip, then immediately flip again we're probably going to get a blank screen, or perhaps just whatever the previous buffer use to have - which might be why it seemed to work in Linux.)

I'll do a bit more testing, but my current recommendation is to rename Zoom::flipWithZoom to Zoom::blitWithZoom or something like that, and get remove its call to SDL_GL_SwapBuffers(); then just keep the change that I made in Screen::flip() (which is to either use SDL_GL_SwapBuffers() for OpenGL mode, or SDL_Flip(_screen) for ordinary 2D SDL mode.)

There are still a few things I'm foggy on. For example, I don't fully understand the purpose of OpenGL::refresh(). The description says "make the buffer show up on screen"; which makes it sound exactly the same as Flip and SwapBuffers... so that's confusing to me.

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Ok. I've amended my original fix to do what I said in the previous post. Here it is:
karadoc@7138594

(I figure that since no one relies on my repository, I can just force history changing commits for neatness.)

I've tested this (briefly) on both Windows and Linux, with and without shaders. I did not notice any problems related to that. So I think this time is is properly fixed!

That said, one time during my testing I did get a crash in Windows while switching shaders. It wasn't with the final code that I'm uploading now, but it was with a version without the Screen::flip(). Essentially what I'm saying is that I don't think my patch here is going to fix #1389 after all. I now believe that's something unrelated. I haven't been able to reliably reproduce the crash. It just seems to sometimes happen when switching between shaders.

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