Fix blend render returning empty result

[dmitrylyzo]

libass may return empty images, and 'blendResult' may become invalid (with zero height), leading to a runtime error when displaying.
Noticed on 'Railgun S Karaoke Test' ~26.38s (after the first verse) with dropAllAnimations enabled.

Would checking <= 0 be better, since width and height are int?
They probably can't be negative by design, but... 🤔

This seems to be reproduced with wasm-blend on empty_image.ass.txt

[TheOneric]

Thanks! Can you be more verbose in your description on why and how exactly a zero width image leads to runtime errors later?

Would checking <= 0 be better, since width and height are int?
They probably can't be negative by design, but... 🤔

Internally the positions (but not the extnents afaik) can be negative, but before the images are returned to the API-user libass clips them to the positive range. Negative values being returned would be a libass bug and this is also relied upon by mpv. So this is safe.

On a side note, I find it a bit odd to always subtract 1 just to later readd 1. If it's to avoid signed-overflows, imho using unsigned is the better option. Either way this isn't directly harmful and unrelated to the bug being addressed here.

[dmitrylyzo]

Uncaught DOMException: Failed to construct 'ImageData': The source height is zero or not a number.

Internally the positions (but not the extnents afaik) can be negative, but before the images are returned to the API-user libass clips them to the positive range. Negative values being returned would be a libass bug and this is also relied upon by mpv. So this is safe.

👍

On a side note, I find it a bit odd to always subtract 1 just to later readd 1. If it's to avoid signed-overflows, imho using unsigned is the better option. Either way this isn't directly harmful and unrelated to the bug being addressed here.

I think this is to get the exact right pixel: x + w - 1 because x + w is an outer pixel.
I haven't looked too deep, you are probably right and +-1 can be safely removed.

[TheOneric]

Uncaught DOMException: Failed to construct 'ImageData': The source height is zero or not a number.

[In the commit message:] [...] leading to a runtime error when displaying, being unable to create an image with zero or invalid size.

I think the commit message could still be a bit more explicit naming what API can't create zero-area images, e.g.:

libass may return empty images, but browsers do not allow
creation of zero-area ImageData objects. Skipping and
exiting early on empty images avoids this and as a bonus
also saves a few CPU-cycles.

Initially noticed on the 'Railgun S Karaoke Test' sample
at ~26.38s (after the first verse) with `dropAllAnimations`
from jellyfin enabled.

Otherwise this seems good to merge to me, thanks.