mjpeg and jpeg encoder: fix alignment warnings

As the input line could be uint8_t*, uint16_t* or uint32_t*, changing
the default from uint8_t* to void* seems the correct choice to deal with
upcasting warnings.

Regarding chunks->data allocation, I quote Frediano explantion:
"Lines came from spice_bitmap_get_line. This function assume that bitmap
data is split among chunks each containing some lines
(always full lines). If chunk->data is allocated using malloc or similar
SHOULD (not 100% sure) be 4 bytes aligned so in our cases
(8, 16, 24 or 32 bit images) should be aligned enough.

All the casts unfortunately came from the fact we compute based on
pixel bytes to make it generic so we use uint8_t*."

and

"Looking at code looks like these chunks came from the virtual machine.
So the question is... why should the virtual machine give use some
not-pixel align data?
I would put a large comment to state that we assume VM send aligned
data, would be stupid for the VM to not align it!"

clang output:
jpeg_encoder.c:109:26: error: cast from 'uint8_t *'
(aka 'unsigned char *') to 'uint16_t *' (aka 'unsigned short *')
increases required alignment from 1 to 2 [-Werror,-Wcast-align]
  uint16_t *src_line = (uint16_t *)line;
                       ^~~~~~~~~~~~~~~~

jpeg_encoder.c:144:26: error: cast from 'uint8_t *'
(aka 'unsigned char *') to 'uint32_t *' (aka 'unsigned int *')
increases required alignment from 1 to 4 [-Werror,-Wcast-align]
  uint32_t *src_line = (uint32_t *)line;
                       ^~~~~~~~~~~~~~~~

mjpeg_encoder.c:260:23: error: cast from 'uint8_t *'
(aka 'unsigned char *') to 'uint16_t *' (aka 'unsigned short *')
increases required alignment from 1 to 2 [-Werror,-Wcast-align]
  uint16_t pixel = *(uint16_t *)src;
                    ^~~~~~~~~~~~~~~

0 files changed, 0 insertions, 0 deletions