pattern_grid: explicit side through PatternGrid::size

The grid's preferred size was the theme's fluid constant (264 px at the reference surface) capped by the width the parent allocates, so a caller could only shrink it: a lock screen that wants the grid to span most of a phone's width had no way to ask for it, and a container's max_width did nothing past the default. PatternGrid gains a size field with a size( impl Into<Length> ) builder; preferred_size resolves it through Canvas::resolve_geom like the other geometry overrides and still caps it at max_width, and map_msg carries it across. Without the builder the fluid default applies as before.
docs/widgets.md describes the builder next to hide_stroke and error; CHANGELOG lists it under Unreleased.

src/widget/pattern_grid/mod.rs

@@ -8,7 +8,7 @@
 //! `on_release`.
 
 use std::sync::Arc;
-use crate::types::{ Color, Point, Rect };
+use crate::types::{ Color, Length, Point, Rect };
 use crate::render::Canvas;
 use super::Element;
 
@@ -25,13 +25,21 @@ pub struct PatternGrid<Msg: Clone>
 	pub( crate ) on_release:  Option<Msg>,
 	pub( crate ) hide_stroke: bool,
 	pub( crate ) error:       bool,
+	pub( crate ) size:        Option<Length>,
 }
 
 impl<Msg: Clone> PatternGrid<Msg>
 {
 	pub fn new( path: Vec<u8> ) -> Self
 	{
-		Self { path, on_node: None, on_release: None, hide_stroke: false, error: false }
+		Self { path, on_node: None, on_release: None, hide_stroke: false, error: false, size: None }
+	}
+
+	/// Side of the square grid; without it the theme's fluid default applies.
+	pub fn size( mut self, s: impl Into<Length> ) -> Self
+	{
+		self.size = Some( s.into() );
+		self
 	}
 
 	pub fn on_node( mut self, f: impl Fn( u8 ) -> Msg + 'static ) -> Self
@@ -77,12 +85,18 @@ impl<Msg: Clone> PatternGrid<Msg>
 			on_release:  self.on_release.map( |m| ( *f )( m ) ),
 			hide_stroke: self.hide_stroke,
 			error:       self.error,
+			size:        self.size,
 		}
 	}
 
 	pub fn preferred_size( &self, max_width: f32, canvas: &Canvas ) -> ( f32, f32 )
 	{
-		let size = canvas.geom_px( theme::SIZE ).min( max_width );
+		let size = match self.size
+		{
+			Some( l ) => canvas.resolve_geom( l ),
+			None      => canvas.geom_px( theme::SIZE ),
+		};
+		let size = size.min( max_width );
 		( size, size )
 	}