We had one implementation of this type of data structure in Arm64Emitter
and one in VideoCommon. This moves the Arm64Emitter implementation to
its own file and adds begin and end functions to it, so that VideoCommon
can use it.

You may notice that the license header for the new file is CC0. I wrote
the Arm64Emitter implementation of SmallVector, so this should be no
problem.

There wasn't really a good place for it, but this will do

And fix bug where opengl was getting the wrong coordinates

Almost all the virtual functions in Renderer are part of dolphin's
"graphics api abstraction layer", which has slowly formed over the
last decade or two.

Most of the work was done previously with the introduction of the
various "AbstractX" classes, associated with texture cache cleanups
and implementation of newer graphics APIs (Direct3D 12, Vulkan, Metal).
We are simply taking the last step and yeeting these functions out
of Renderer.

This "AbstractGfx" class is now completely agnostic of any details
from the flipper/hollywood GPU we are emulating, though somewhat
specialized.

(Will not build, this commit only contains changes outside VideoBackends)

This increases accuracy, fixing the white rendering in Major Minor's Majestic March.  However, the hardware backends can only have one viewport and scissor rectangle at a time, while sometimes multiple are needed to accurately emulate what is happening.  If possible, this will need to be fixed later.