VideoCommon/BoundingBox: Make interface for querying bounding box data

Rather than expose the bounding box members directly, we can instead
provide an interface for code to use. This makes it nicer to transition
from global data, as the interface function names are already in
place.

Source/Core/VideoCommon/PixelEngine.cpp

@@ -233,7 +233,7 @@ void RegisterMMIO(MMIO::Mapping* mmio, u32 base)
   for (int i = 0; i < 4; ++i)
   {
     mmio->Register(base | (PE_BBOX_LEFT + 2 * i), MMIO::ComplexRead<u16>([i](u32) {
-                     BoundingBox::active = false;
+                     BoundingBox::Disable();
                      PixelShaderManager::SetBoundingBoxActive(false);
                      return g_video_backend->Video_GetBoundingBox(i);
                    }),