Check for buffer overflow in prerendered variant

[embediver]

In the prerendered variant one can pass a iterator with more elements than LEDs, which leads to a buffer overflow.

This PR fixes this by adding a new error type for the prerendered variant.

pub enum Error<E> {
    OutOfBounds,
    Spi(E),
}

Now the current index will be checked in write_byte() and return an OutOfBounds error if it will exceed the data buffer size.

[david-sawatzke]

Thank you, this looks good!