Check format & modifier

[emersion]

Before trying an atomic test commit, check the format & modifier is compatible.

It's not clear it's a good idea yet: all we get is a FB ID. We can drmModeGetFB it, but then we don't save an ioctl. If we cache the info, it may become out-of-date (e.g. FB deleted and re-created with another format).

Maybe we can just get the format/modifier from the library user. Need to check whether that would really improve performance though.

[emersion]

but then we don't save an ioctl

Actually, if we only drmModeGetFB once at the beginning of the plane alloc (or lazily), then we could save multiple atomic test commits.

[emersion]

Unfortunately drmModeGetFB doesn't provide the format/modifier. We'll need to get it from the compositor (liftoff_layer_set_fb?), which we may have ended up doing anyway to avoid getting from the kernel information owned by the compositor itself.

[emersion]

There's a patch for getFB2: https://patchwork.freedesktop.org/patch/334263/?series=67552&rev=1

Still not sure it's a good idea to use it.