png is wrong

I downloaded the fb2png and execute

```
cd /data/local
./fb2png fbdump.png
```

and I got the output

```
root@android:/data/local # ./fb2png fbdump.png
./fb2png fbdump.png
bpp : 4
width : 1080
height : 1920
pixel : 8294400
r_offset : 24
g_offset : 16
b_offset : 8
a_offset : 0
r_length : 8
g_length : 8
b_length : 8
a_length : 8
OKAY
```

after that I pulled the fbdump.png and look, but it seems png not convert right.

My phone is ZTE v988,

Reproduce the `./fb2png fbdump.png` command from `/data/local` on the reported ZTE v988 and inspect the generated `fbdump.png` alongside the displayed bpp, dimensions, and channel offsets. Start at the `fb2png` executable's framebuffer-format handling; done means the captured PNG renders correctly on this device and the issue includes enough output to verify the result.

Written by the indexing model from the issue text.