Implement FrameBufferBackend for slices

[jounathaen]

This is useful, e.g., if the framebuffer is declared by another entity and is passed to the application as a pointer and a size.

[svenstaro]

Should we add a note somewhere that this might not work with DMA if the buffer lives in PSRAM connected via SPI (which is then by its nature not DMA)?

[jounathaen]

Hmm, this might be a problem indeed. However, I think this issue persisted before and I don't think this is related to this PR.

I'm not so deep into this code anymore, but isn't what we do, that we basically implement ReadBuffer for all &mut [C;N] in

embedded-graphics-framebuf/src/backends.rs

Line 92 in 4205fb2

unsafe impl<'a, C: PixelColor, const N: usize> DMACapableFrameBufferBackend for &'a mut [C; N] {

, but we have a note that the requirements from https://docs.rs/embedded-dma/latest/embedded_dma/trait.ReadBuffer.html still apply. The rules there don't state that it needs to be DMA enabled memory, indeed.

Maybe you have a better idea, but my suggestion would be to add this to embedded_dma.

[svenstaro]

So, I think this PR by itself is totally fine and fulfills my feature request in #22. DMA not being possible with SPRAM via SPI is an orthogonal issue.

[jounathaen]

@bernii With two "approvals", I take the freedom to merge this now 🙂