Fix erroneous scroll when using X/Y offsets in images - #10256

Problem
-------

In update_dest_rect() when both num_cols and num_rows are 0
(auto-computed, ``a=T``), the first block computes num_cols with
cell_x_offset, but, then the second block mismatches num_cols, causing
it to compute width_px from num_cols and adds cell_x_offset a second
time. The returned value is large enough that it often causes scrolling,
depending on size and location of the screen.

A sample visual program:

```python
import zlib, base64, time
from functools import partial
echo = partial(print, end='', flush=True)

echo('\033[H\033[J')

base = bytes([0, 128, 0, 255]) * 640 * 576
k = zlib.compress(keyframe, 3)
echo(f'\033[8;60H\033_Ga=T,i=1,q=1,f=32,s=640,v=576,o=z,N=1;{base64.b64encode(k).decode()}\033\\')
time.sleep(1)

delta = bytes([200, 0, 0, 255]) * 32 * 512
d = zlib.compress(delta, 3)
echo(f'\033[9;67H\033_Ga=T,i=100,q=1,f=32,s=32,v=512,o=z,N=1,X=9,Y=25;{base64.b64encode(d).decode()}\033\\')

time.sleep(1)
print()
```

Solution
--------

Add 'auto_cols' and 'auto_rows' to remember the original read-only
values before exercising their auto-calculated size. 'auto_rows' isn't
technically necessary given the logic branching but it describes in code
better.

Before and After video
----------------------

A video will be attached shortly, here.