gl_rasterizer: Port front face flip check from Vulkan

While Vulkan was assuming we had no negative viewports, OpenGL code
was assuming we had them. Port the old code from Vulkan to OpenGL,
checking if the first viewport is negative before flipping faces.

This is not a complete implementation since we only check for the first
viewport to be negative. That said, unless a game is using Vulkan,
OpenGL and NVN games should be fine here, and we can always compare with
our Vulkan backend to see if there's a difference.