fix: correct text alpha blending

[var4yn]

FIx draw text with alpha.

The previous version of the function added a white outline to the text. When attempting to render semi-transparent text (e.g., using alpha = 125), the text ended up cutting out the background instead of blending with it.

See the examples below:
— The top label is rendered using the old function.
— The bottom label shows the result after my changes.

• rgb alpha 255 text

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• rgb alpha 125 text ( old function cuts the image )

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• luma alpha 225 text

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• luma alpha 125 text

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Suggestions:

To compute a has_alpha: bool flag, I based my logic on this.
It may be useful to expose this logic via Pixel::has_alpha() method inside the image crate