Reduce memory needed to open PNG images

Fixes http://trac.openmicroscopy.org/ome/ticket/12274.

To test, change the ```BF_MAX_MEM``` variable in ```tools/bf.sh``` or ```tools/bf.bat``` to ```768m```.  Verify that the following two commands on the file in ```from_skyking/png/james``` result in an OutOfMemoryError without this change:

```showinf -crop 0,0,512,512``` -- this is the first tile
```showinf -crop 21921,21970,512,512``` - this is the last tile

With this change, both commands should result in the tile being displayed; it is expected for both tiles to be all black.  Reading the last tile will be noticeably slower than reading the first tile, which is also expected.

All builds should remain green.