Improve panic messages when providing invalid buffer sizes to ImageEncoders

[MrGunflame]

Replaces the cryptic assertion failed into a more useful panic message when writing a PNG image with invalid buffer/dimensions.

[fintelia]

Thanks for this suggestion, I think a clearer panic message would be an improvement. One note is that you don't actually have to switch to if ... { panic!(...) } since there's a version of the assert_eq! macro that takes a format string as its third argument.

As far as the message itself, I think we might be able to be a bit more concise. Perhaps "Invalid buffer length: expected {} got {}"? The PNG encoding part should be evident from the backtrace into the PngEncoder::write_image` method.

Finally, this same assertion is actually present in all the other ImageEncoder implementations as well, so I think it would make sense to update them all at the same time.

[MrGunflame]

As far as the message itself, I think we might be able to be a bit more concise. Perhaps "Invalid buffer length: expected {} got {}"? The PNG encoding part should be evident from the backtrace into the PngEncoder::write_image` method.

I do think it would be helpful to include how the buffer size was derived, i.e. include width and height and maybe even ColorType as that info makes tracking down the panic easier IMO.

[MrGunflame]

I think that includes all encoders now. I've also attempted to make the panic message more concise, but if including the width/height is too verbose I'm also fine removing them again.

[kornelski]

Maybe add #[track_caller] on these functions, since the assert is not an internal bug, but the fault of the caller.

[fintelia]

Thanks!