Linus Groh e4bba2fab9 LibGfx: Keep alpha value of previous pixel for QOI_OP_RGB chunk ...

I accidentally skipped this part of the spec in the QOI decoder:

> The alpha value remains unchanged from the previous pixel.

This led to incorrect rendering of some images with transparency,
visible in form of a horizontal line of non-transparent pixels (that
shouldn't exist), e.g. for the following chunk sequence:

- QOI_OP_RGBA with alpha = 0
- QOI_OP_RGB
- QOI_OP_RUN

The QOI_OP_RGB should 'inherit' the alpha value of the previous
QOI_OP_RGBA chunk, instead of always setting it to 255.
I'm unsure why the encoder added the QOI_OP_RGB chunk to the specific
image where the bug was noticed in the first place - they effectively
both had fully transparent color values.

2021-12-22 01:12:51 +01:00

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