vsync in linux can't be set

sdlayer_checkvsync does some magic to determine if vsync is locked. Later it prevents videoSetVsync from change vsync options at all. However if i remove first check with vsync_unsupported i can have tear-free image.

I suspect that SDL_GL_GetSwapInterval reports either "adaptive sync" or "no sync" and this most likely breaks sdlayer_checkvsync magic and disable vsync switching at all.