image.c is now completely OpenGL independent.  The rendering callbacks
now pass the parameters that IceT fiddles with (matrices, background,
readback region) and lets the user code render with whatever system
available.  The OpenGL layer now has the code that sets up OpenGL before
making its own callback (which, like before, takes no arguments) and
then reading the data back from the OpenGL buffers.

I know that there are glaring bugs in this code.  I will address them
soon.