Replace hardcoded u8 by the Pixel trait to allow 10 bit signals

In the current implementation, 10-bit signals can be stored, but they are derived from 8-bit signals, offering no visual improvement. All incoming data has been assumed to be 8-bit sRGB. This PR takes the first step in removing these assumptions by enabling the storage of signals with higher bit depth. It lays the groundwork for full support of writing HDR images.