Mirror memory for VRAM mirrors.

For a long time we've done this with GetPointer() only, but it's better if
it's the same for all methods of memory access.

Technically, we should do the swizzling (and also swizzle the depth data in
softgpu), but there's no indication games depend on this, and GetPointer
would need changes anyway.
Merge pull request #6037 from unknownbrackets/memmap

Mirror memory for VRAM mirrors