Fix `WaitForNextFrame` (legacy and modern) for NVidia

Here's a lead: https://www.gamedev.net/forums/topic/681754-why-so-many-fencesync-types/?do=findComment&comment=5308536 and https://www.khronos.org/registry/OpenGL/extensions/NV/NV_fence.txt

This is the best sense I can make out of it:

if( glewIsSupported("GL_NV_fence") )  glGenFencesNV( 1, &fenceNV );
// ...
if( fenceNV )
{
     glSetFence( fenceNV );
     glFinishFenceNV( fenceNV, GL_ALL_COMPLETED_NV );
}
// ...
if( fence ) glDeletesFenceNV( 1, &fenceNV );

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