VideoCommon: Account for pixel quads in bounding box calculation

The GC/Wii GPU rasterizes in 2x2 pixel groups, so bounding box values
will be rounded to the extents of these groups, rather than the exact
pixel. To account for this, we'll round the top/left down to even and
the bottom/right up to odd. I have verified that the values resulting
from this change exactly match a real Wii.

Source/Core/VideoCommon/RenderBase.cpp

@@ -187,7 +187,18 @@ void Renderer::ReinterpretPixelData(EFBReinterpretType convtype)
 
 u16 Renderer::BBoxRead(int index)
 {
-  return BBoxReadImpl(index);
+  u16 value = BBoxReadImpl(index);
+
+  // The GC/Wii GPU rasterizes in 2x2 pixel groups, so bounding box values will be rounded to the
+  // extents of these groups, rather than the exact pixel.
+  // This would have been handled in the pixel shader, but all attempts to do so did not work on
+  // OpenGL/NVIDIA, due to presumably mystical driver behavior with atomics.
+  if (index == 0 || index == 2)
+    value &= ~1;
+  else
+    value |= 1;
+
+  return value;
 }
 
 void Renderer::BBoxWrite(int index, u16 value)