Happened to find this when working on the previous commit.

Now only VertexLoader remains... But that one might be tricky.

This fourth part of my series of patches to get rid of unsafe uses of
GetPointer takes care of the "easy" cases in VideoCommon. Three uses of
GetPointer now remain in Dolphin: VertexLoaderManager, TextureInfo, and
the software renderer's TextureSampler.

Instead, we make the event take a reference to the system and then pass
it in when the event is triggered.

This does introduce two other accessors, but these are much easier to
refactor out over time, and without modification to the existing event
interface.

Simplifies the interface in terms of usage