[GLExtensions] Improve the extension loader.

This saves allocating a vector for the pass-through path.

This was relying on behaviour that GLExtensions was adding fake extensions to the supported list with ES.
This no longer happens so it needed to be changed.

It's fine to pull those within the backends, so there is no need to enforce them to implement this interface.

This adds a check to make sure the vendor supports anisotropic filtering.
Pretty much all vendors support this, even the mobile ones.

It was only implemented in OpenGL, though the option was visible in both
backends, leading to memory leaks if you enabled it in DirectX.

And it wasn't particularly useful as a debug feature as it only showed
where in the EFB the copies were taken from, not what format it was, or
what the copy was used for, or what content was in the EFB at that point
in time.

Also, it stretched the copy regions relative to the window, so the
on-screen regions don't even line up with the window unless the game used
the full EFB (some pal games) and you game image stretched to the full
window.

Allows Mesa based drivers to support ZCompLoc

SSAA relies on MSAA being active to work. We only supports 4x SSAA while in fact you can enable SSAA at any MSAA level.
I even managed to run 64xMSAA + SSAA on my Quadro which made some pretty sleek looking games. They were very cinematic though.

With this, it properly fixes up SSAA and MSAA support in GLES as well. Before they were broken when stereo rendering was enabled.
Now in GLES they can properly support MSAA and also stereo rendering with MSAA enabled(with proper extensions).