[ltk — src/input/gesture/mod.rs, src/input/gesture/tests.rs, src/widget/handlers.rs, src/input/keyboard/dispatch.rs, src/render/image.rs, docs/architecture.md, CHANGELOG.md] Keep a tap on a button while the finger rolls inside it, treat XF86Back like Escape, and convert ARGB8888 by whole pixels

Any travel past the slop turned a press into a vertical or horizontal drag and its release skipped the button, so a thumb that rolled on a large key lost the tap. `GestureState::on_move` now stays idle while the pressed widget fires on press (the new `WidgetHandlers::has_press`) and the finger is still inside its rect; the swipe machinery takes over from the original press point once the finger leaves. Presses inside a scroll viewport are unaffected, since a list of full-width rows is scrolled by dragging on them. Two tests cover the tap that survives motion inside the button and the swipe that arms once the finger leaves it; the architecture notes and the changelog describe the rule.
The compositor now sends XF86Back for the Android back gesture, an inward swipe from either side edge. With a text input focused that keysym fell into `handle_key_default` and was swallowed, so an app never saw it. `XF86Back` now dispatches through `handle_key_escape`, so every ltk app gets the back gesture with Escape's layering: open popups, then the context menu, then a selection, then a dialog's cancel, and finally the focus drop and `on_key_with_modifiers` with the keysym, where an app can act on it.
`SoftwareCanvas::write_to_wayland_buf` swapped red and blue with a byte-by-byte loop, unrolled by hand over sixteen bytes. It now reads each pixel as a `u32`, exchanges the two channels with masks and shifts, and leaves the vectorisation to the compiler. Compositors that offer `ABGR8888` never took this path.

src/widget/handlers.rs

@@ -295,6 +295,12 @@ impl<Msg: Clone> WidgetHandlers<Msg>
 		}
 	}
 
+	/// Whether a tap on this widget fires something.
+	pub fn has_press( &self ) -> bool
+	{
+		self.press_msg().is_some()
+	}
+
 	/// Submit message (Enter on a focused TextEdit). `None` for every other
 	/// variant.
 	pub fn submit_msg( &self ) -> Option<Msg>