Fix erroneous scroll when using X/Y offsets in images

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.

kitty_tests/graphics.py

@@ -647,6 +647,13 @@ class TestGraphics(BaseTest):
         self.ae((s.cursor.x, s.cursor.y), (3, 2))
         rect_eq(layers(s)[0]['dest_rect'], -1, 1, -1 + 3 * dx, 1 - 3*dy)
 
+    def test_graphics_put_with_pixel_offsets(self):
+        cw, ch = 10, 20
+        # Image 10x20 placed with 5px X and Y pixel offsets
+        s, dx, dy, put_image, put_ref, layers, rect_eq = put_helpers(self, cw, ch)
+        self.ae(put_image(s, 10, 20, cell_x_off=5, cell_y_off=5)[1], 'OK')
+        self.ae((s.cursor.x, s.cursor.y), (2, 1))
+
     def test_image_layer_grouping(self):
         cw, ch = 10, 20
         s, dx, dy, put_image, put_ref, layers, rect_eq = put_helpers(self, cw, ch)