render: pre-scale font_line_metrics by dpi_scale

The text pipeline works in physical pixels: measure_text and draw_text multiply the font size by dpi_scale internally, and the layout rects widgets receive are physical too. font_line_metrics was the one exception — it returned line metrics at the logical size — so the line height and ascent the text and rich_text widgets derive from it came out divided by the surface scale factor. On any output with a scale other than 1 (e.g. a 200% display, or any fractional setting the compositor rounds up to buffer scale 2), wrapped lines overlapped, baselines sat too high, and preferred_size reported half the real text height.
Scale the size handed to horizontal_line_metrics by dpi_scale in both the Canvas wrapper and the GLES backend, matching the already pre-scaled font_metrics. At scale 1 the behaviour is unchanged.

src/render/mod.rs

@@ -529,10 +529,11 @@ impl Canvas
 		self.font().metrics( ch, size * self.dpi_scale() )
 	}
 
-	/// Convenience wrapper around `font().horizontal_line_metrics(...)`.
+	/// Convenience wrapper around `font().horizontal_line_metrics(...)`,
+	/// pre-scaled by `dpi_scale` like [`Self::font_metrics`].
 	pub fn font_line_metrics( &self, size: f32 ) -> Option<LineMetrics>
 	{
-		self.font().horizontal_line_metrics( size )
+		self.font().horizontal_line_metrics( size * self.dpi_scale() )
 	}
 
 	pub fn resize( &mut self, width: u32, height: u32 )