expo: Rename scene_dim to scene_obj_bbox

expo: Rename scene_dim to scene_obj_bbox

At present we assume that each object is a simple box and that it fills
the whole box.

This is quite limiting for text objects, which we may want to centre
within the box. We need a position within the box where drawing starts.

Rename the scene_dim struct to indicate that it is a bounding box.

Signed-off-by: Simon Glass <sjg@chromium.org>