[GLES] Fix for SBS rendering and cached viewport

I've also changed the unused function CLinuxRendererGLES::RenderMultiPass
to use the caching wrapper for SetViewPort.
Makes no odds now, but may avoid a problem if it's enabled in future.
Merge pull request #3754 from popcornmix/fix_viewport_cache

[GLES] Fix for SBS rendering and cached viewport
[nfs] - get rid of some CStdStrings in favour of std::string
[nfs] - better fix for fe97d68f87d6985b2bf57d8e942b36b5e7373066 - possibly fixes #14727 - thx jm for the idea :)