yoda: softbuffer + window_clipboard default-features=false

Two one-line workspace-dep fixes that drop the last remaining X11 crates
for Wayland-only builds:

- softbuffer: its default = [kms, x11, x11-dlopen, wayland, wayland-dlopen]
  pulled tiny-xlib + as-raw-xcb-connection unconditionally even though
  iced_tiny_skia only needed softbuffer/wayland. Disabling the default
  lets iced_tiny_skia's own feature propagation ask for just wayland +
  wayland-dlopen.

- window_clipboard: upstream pulled clipboard_x11 + clipboard_wayland
  unconditionally on unix. Our local fork (branch yoda-x11-optional)
  gates clipboard_x11 behind an opt-in feature, so default-features=false
  + features=["wayland"] drops ~500 x11 symbols (clipboard_x11, x11rb,
  x11rb-protocol).

Combined with previous yoda-v4 cuts, cosmic-yoterm goes from
1526 -> 43 x11 symbols (-97%), and 55.4 MB -> 53.3 MB (-2.1 MB).

Cargo.toml

@@ -276,7 +276,10 @@ tiny-skia = { version = "0.11", default-features = false, features = [
     "simd",
 ] }
 cctk = { git = "https://github.com/pop-os/cosmic-protocols", package = "cosmic-client-toolkit", rev = "160b086" }
-softbuffer = { git = "https://github.com/pop-os/softbuffer", tag = "cosmic-4.0" }
+# Yoda: default-features=false drops softbuffer's x11 / x11-dlopen (pulled
+# tiny-xlib + as-raw-xcb-connection). The wayland + wayland-dlopen features
+# are re-activated via iced_tiny_skia's own `wayland` feature propagation.
+softbuffer = { git = "https://github.com/pop-os/softbuffer", tag = "cosmic-4.0", default-features = false }
 syntect = "5.2"
 tokio = "1.0"
 tracing = "0.1"
@@ -302,7 +305,11 @@ wayland-client = { version = "0.31.5" }
 winapi = "0.3"
 # window_clipboard = "0.4.1"
 
-window_clipboard = { git = "https://github.com/pop-os/window_clipboard.git", tag = "sctk-0.20" }
+# Yoda: window_clipboard via git path = our local fork that has x11 gated
+# behind an opt-in feature. default-features=false + features=["wayland"]
+# drops clipboard_x11 + x11rb from the build.
+# Kept at /home/lionel/Devels/window_clipboard branch yoda-x11-optional.
+window_clipboard = { git = "https://github.com/pop-os/window_clipboard.git", tag = "sctk-0.20", default-features = false, features = ["wayland"] }
 dnd = { git = "https://github.com/pop-os/window_clipboard.git", tag = "sctk-0.20" }
 mime = { git = "https://github.com/pop-os/window_clipboard.git", tag = "sctk-0.20" }
 # Yoda: default-features = false drops winit's x11 default, which otherwise