Wrong Transparency in ImageSurface.get_data

Hello,

It seems that `ImageSurface.get_data` gives different transparency values than `ImageSurface.write_to_png`. It seems as if the one from `get_data` is incorrect.

How to reproduce:
```python
import cairosvg.parser

import numpy as np
import cv2

svg_text = """

"""

tree = cairosvg.parser.Tree(bytestring=svg_text.encode('utf-8'))
surf = cairosvg.surface.PNGSurface(tree, None, dpi=96).cairo
# write to PNG
surf.write_to_png('1_write_to_png.png')
# write to RGBA buffer
data = np.frombuffer(surf.get_data(), np.uint8).reshape(surf.get_height(), surf.get_width(), 4)
cv2.imwrite("2_get_data.png", data)
```

This generates the two PNG's here:

 

The problem is visible when they are viewed on a white background. Zooming in reveals:


The left one is `write_to_png`. The right `get_data`. The black outline on the second image is not correct.

Here is the alpha channel. It looks identical:


And the color channels (without alpha):


It looks like the alpha value is pre-applied to the RGB channel.

Is this just a different convention, as we are using cairo internals and the output needs to be processed before using it as a typical RGBA channel?

I found the snipped with `get_data` in some issues here, maybe it was incomplete?

System information:
```
Ubuntu 21.10
Python 3.8.12
CairoSVG==2.5.2
cairocffi==1.3.0
libcairo2 1.16.0
```

I tested other png writers like the one in `matlab` and `PIL` and get the same output.

Start by running the supplied CairoSVG, cairocffi, NumPy, and OpenCV reproduction, then compare the ImageSurface.get_data and write_to_png entry points on the PNGSurface cairo object. Check the Cairo image-buffer convention and determine whether the difference is expected; done means the behavior is corrected or clearly documented, with coverage for the reproduced transparency case.

Written by the indexing model from the issue text.