bugfix: get rid of memory leak in koptreader

Memory leak came from the master bitmap that is reused in each reflow. But the bitmap
width and height were not set to zero when bitmap was freed. When the master bitmap was
enlarged in some cases the size of the master bitmap would increase monotonically.
This patch fixes this issue by setting master bitmap width and height to zero at the
beginning of each reflow. And a memeory usage logger is added in koptreader.

libk2pdfopt

@@ -1 +1 @@
-Subproject commit e65d9208a06de942ff674a17aeb311b63af275b3
+Subproject commit d9ed4f0b5d9212cdcc97fa432b83ba46eab21d09