Worked around render target bug on macOS

[eXpl0it3r]

Fixes #1132

I thought to open this PR here, so we can discuss if it works or not here and either merge it or reject it. 🙂

@mantognini mentioned

I invite others to test this as well by checking out the bugfix/osx_render_target branch.

@achpile mentioned

this patch didn't fixed my issue.
everything renders normally if there's something drawn on RenderWindow except the sprite with RenderTexture (as in my example code).

[JonnyPtn]

For what it's worth this looks good to me. Using steps from original issue, I get a blank screen on master, and a purple circle on this branch

[mantognini]

Thanks @JonnyPtn for testing this!

@achpile, could you try it again? I'm wondering if you forgot to clear the old binaries before recompiling SFML and trying the bugfix/osx_render_target branch. This has been a common issue with many testers.

In the meantime, since we have no better fix and it seems to fix at least some issues for some users, I'd say we go ahead and merge this. Or do you have another, better fix in mind, @binary1248?

[ghost]

@mantognini sorry, i no longer have macbook :(

[eXpl0it3r]

Since the change is so small and potentially fixes the issue, let's just merge it and if similar issues arise, we can always revert it again.

[binary1248]

@mantognini No idea if there is another fix. During testing I just realized that resetting the states didn't result in any actual change unless you switch contexts back and forth before. This behaviour is totally unexpected and is not documented anywhere. As you noticed yourself, this is a regression since it used to work in older OS X versions. Considering that there is probably not that much software out there that makes such liberal use of multiple contexts like SFML does, I don't expect Apple to go out of their way to fix this any time soon.

[mantognini]

Alright, so let's merge this!