OK, the format of saved meshes in the Sprite object is:
A 4-byte ASCII string saying 'HSEM' ('MESH' backwards because I'm lazy about endian) to identify the MESH file
int32: version integer (always 1 right now, reserved for future use)
int32: horizontal number of points
int32: vertical number of points
Then, repeated for each point (number of times = horizontal points x vertical points), in column order:
float32: x offset (as a fraction of the original image width)
float32: y offset (as a fraction of the original image height)
float32: z elevation
float32: u offset
float32: v offset
float32 x 4: a, r, g, b for point opacity and filter, from 0.0 to 1.0.