Use DMA to flip screen in double buffer "cumulative" mode

[CHiPs44]

cf.

• https://github.com/raspberrypi/pico-examples/blob/master/dma/hello_dma/hello_dma.c
• https://www.raspberrypi.com/documentation/pico-sdk/hardware.html#group_hardware_dma

Prototype:

void * dma_memcpy( void * restrict destination, const void * restrict source, size_t size );

For now, if source and destination do overlap:

• NB: determining memory overlapping can be a challenge: if src points to "striped" sram and dst to "sequential" ram, they could overlap and pointer arithmetic wouldn't notice
• use memcpy()

else:

• use DMA_SIZE_32 if destination % 4 == 0 && source % 4 == 0 && size % 4 == 0
• use DMA_SIZE_16 if destination % 2 == 0 && source % 2 == 0 && size % 2 == 0
• use DMA_SIZE_8 otherwise

[CHiPs44]

see https://github.com/tvlad1234/pico-vgaDisplay for dma_memset and dma_memcpy.