Add bounds properties for each image region

For sparse images, some applications may want the exact bounds of each
region which actually contains pixels.  Define four properties for each
ROI:

    openslide.region[i].x
    openslide.region[i].y
    openslide.region[i].width
    openslide.region[i].height

Provide these properties for Leica and Ventana, where the information is
readily available.  They could also be calculated for MIRAX with some
effort, but we don't do that here.  Leave the properties undocumented
for now.

Discussion:

    http://lists.andrew.cmu.edu/pipermail/openslide-users/2014-November/000972.html

Also relevant to #148.