Misallocated Pixels when GS2HMSBFormat has a width that's not a multiple of 4

When the frame buffer has a width that's not a multiple of 4, pixels at the edge are misallocated.
I've put together the following as an example:

```
BLACK = 1
RED = 2
WHITE = 3

WIDTH = 7
HEIGHT = 2

buffer = bytearray(math.ceil((WIDTH * HEIGHT) / 4))
fb = adafruit_framebuf.FrameBuffer(
buffer, WIDTH, HEIGHT, buf_format=adafruit_framebuf.GS2_HMSB,
)
fb.fill(WHITE)

def print_arr(a):
"Print the byte array as paired bytes"
n = 2
a = ''.join([f'{v:08b}' for v in a])
a = [a[i:i + n] for i in range(0, len(a), n)]
for r in range(HEIGHT):
print(' '.join(a[r*WIDTH:(r+1)*WIDTH]))
print(' '.join(a[(r+1)*WIDTH:]))

color_map = {3:'_', 2: '*', 1: '#'}
def print_buffer(the_fb):
print("+" * (the_fb.width + 2))
for y in range(the_fb.height):
print("+", end="")
for x in range(the_fb.width):
c = fb.pixel(x, y)
print(color_map[c], end="")
print("+")
print("+" * (the_fb.width + 2))
```

Using the above, setup:
```
fb.pixel(4,0, RED)
print_arr(fb.buf)
print_buffer(fb)
```
gives
```
11 11 11 11 11 11 11
10 11 11 11 11 11 11
11 11

+++++++++
+____*__+
+*______+
+++++++++
```
The same result occurs with `fb.pixel(0,1, RED)` as well.

Reproduce the issue with adafruit_framebuf.FrameBuffer using GS2_HMSB, WIDTH=7, HEIGHT=2, and the provided pixel calls. Start by tracing pixel() and the framebuffer packing represented by fb.buf. Done means edge pixels remain in their intended positions for widths that are not multiples of four, including the supplied examples.

Written by the indexing model from the issue text.