wgpu: Fix pixel copying when processing pixebender output

[SuchAFuriousDeath]

Fixes the game mentioned at #12460 ('The End') for me and makes it playable (without the changes, a RangeError is thrown. However, the issue also mentions a separate error that does not occur for me (PR probably upcoming on that one too).

The texture format is Rgba32Float for both 3 and 4 channels, so the channel count should only be taken into account when copying the pixels in each row. Since channels_in_raw_pixels would be a constant, I also refactored the surrounding code a little bit.

[kjarosh]

I think crafting a test for this should be pretty simple