0.5.2_pre3

• Fix memory leak when exporting too often (only an issue with PNG export, bug has existed almost since the beginning)

Although this bug is really bad, it's been around for an extremely long time. I haven't noticed it until recently because memory pressure has been increasing with the webserver and other things. Although the fix was easy, I think I want to redesign some of the large memory use to point to a reusable scratch space, since most of this stuff doesn't need to happen at the same time. For instance, the export buffer can share the same space as the webserver receive buffer, among other things