Support heap allocation in FrameBuffer

[nishanthkarthik]

• Version of embedded-graphics in use (if applicable): 0.8.1

Description of the problem/feature request/other

FrameBuffer embeds a large array data: [u8; N]. Box::new(FrameBuffer::new()) still constructs the frame buffer on the stack before moving it to a box. Ideally I should be able to provide my own backing buffer on the heap.

As a workaround, I can use alloc with the correct layout and use a Box::from_raw.