> rot_offset = 0.5 = rotate about center, rot offset 0. = rotate about top/left, 1.0 = rotate about bottom/right ?

I'll add rotation anchor point in this way. Note that after this "x"
and "y" are not source image top left but anchor point position.
Default values are going to be 0 so default position for anchor point
is going to be source top left. This is the most common way to do it.

To allow for arbitrary image manipulation and animation I'm still
going to have to interpret "x "and "y" like:

0 = anchor point 2 frame widths left/up
0.2 = anchor point 1 frame width left/up
0.4 = anchor point aligned with dest image
0.6 = anchor point just out of dest image bottom/right corner
0.8 = anchor point 1 frame width down/right
1.0 = anchor point 2 frame widths down/right