Works on OSX, not Windows

I have a program that's intended to display the output of a scientific
camera -- a ~2500x~2000 greyscale image. This exceeds the maximum
texture size on the target computer, so I split the image into smaller
tiles which are displayed side-by-side. I wrote up a test application
which works fine on my OSX laptop, but not on the (Windows 7) target
computer. The problem I'm seeing is that every tile is showing what
the last tile is supposed to show. So for example if I render tiles at
(0, 0), (0, 1), (1, 0), and (1, 1), then all of the tiles will display
the content from the (1, 1) tile.

Here's the test app: http://pastebin.com/yEaeLUNv
It depends on wxPython, pyOpenGL, and numpy. Everything up to line 121
is WX glue code and can be ignored (the sample image is generated at
line 10, if you care). The OnPaint function there calls the render()
method for each Image class instance; those Images then render
individual tiles. I apologize for the inconsistent code style and use
of deprecated OpenGL features; this is a mishmash of old and new-ish
code from multiple authors.

Any ideas what I'm doing wrong? Or how I'd go about figuring this out?
I've yet to figure out how to effectively debug errors in my usage of
OpenGL.

-Chris

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