clients: Let simple-shm create buffers of requested RGB format

This is a temporary solution to easily test wl_shm support for
different RGB formats. I think it would be better to have a dedicated
client to test all the supported RGB and YUV formats for the wl_shm,
dma-buf import and legacy EGL protocols.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>