linux-dmabuf: figure out a good way to broadcast set of supported formats/modifiers

On my AMD machine linux-dmabuf advertises 310 format/modifier pairs. That's 310 * (4 + 4 + 8) = 4960 bytes worth of data being transferred each time a client connects (message header + format + modifier).

This is about to get worse with !8 (merged).

Using a wl_array to send one format and multiple modifiers doesn't help too much: each of the 41 formats has ~7.56 modifiers, so the total size is 41 * (4 + 4 + 4 + 8*7.56) = 2972 bytes (message header + format + array size + modifiers).