OGL: Use a fixed VAO for attributeless rendering.

Instead of abusing whatever VAO is previously bound, which might have
enabled arrays.

Only used in one instance currently, which fixes a crash with older
NVIDIA drivers.
OGL: Properly reset the attributeless VAO.
OGL: Bind the attributeless VAO before EFB copies.

Fixes crashes in Zack & Wiki using an older NVIDIA driver.
OGL: Move attributeless VAO creation to Init.

This way we won't trash an existing bound VBO by mistake.
OGL: Ensure a VAO is bound for all attributeless.

Unfortunately, some of these cases are not well tested, because I don't
know how to reproduce them.
Merge pull request #1650 from unknownbrackets/attributeless-fix

OGL: Unbind the active VAO before attributeless rendering