Use a scratch buffer to reduce allocations

This commit adds a new ShapeBuffer type, which contains some buffers tha
are commonly re-created during the shaping process. New APIs are added
that take this structure, and old APIs are turned into wrappers around
the new API.

The goal is to reduce the number of per-layout allocations that happen
in a typical layout call.

src/buffer.rs

@@ -10,7 +10,7 @@ use unicode_segmentation::UnicodeSegmentation;
 
 use crate::{
     Attrs, AttrsList, BidiParagraphs, BorrowedWithFontSystem, BufferLine, Color, FontSystem,
-    LayoutGlyph, LayoutLine, ShapeLine, Shaping, Wrap,
+    LayoutGlyph, LayoutLine, ShapeBuffer, ShapeLine, Shaping, Wrap,
 };
 
 /// Current cursor location
@@ -330,6 +330,9 @@ pub struct Buffer {
     /// True if a redraw is requires. Set to false after processing
     redraw: bool,
     wrap: Wrap,
+
+    /// Scratch buffer for shaping and laying out.
+    scratch: ShapeBuffer,
 }
 
 impl Buffer {
@@ -354,6 +357,7 @@ impl Buffer {
             scroll: 0,
             redraw: false,
             wrap: Wrap::Word,
+            scratch: ShapeBuffer::default(),
         }
     }
 
@@ -411,7 +415,13 @@ impl Buffer {
             if line.shape_opt().is_none() {
                 reshaped += 1;
             }
-            let layout = line.layout(font_system, self.metrics.font_size, self.width, self.wrap);
+            let layout = line.layout_in_buffer(
+                &mut self.scratch,
+                font_system,
+                self.metrics.font_size,
+                self.width,
+                self.wrap,
+            );
             total_layout += layout.len() as i32;
         }
 
@@ -439,7 +449,13 @@ impl Buffer {
             if line.shape_opt().is_none() {
                 reshaped += 1;
             }
-            let layout = line.layout(font_system, self.metrics.font_size, self.width, self.wrap);
+            let layout = line.layout_in_buffer(
+                &mut self.scratch,
+                font_system,
+                self.metrics.font_size,
+                self.width,
+                self.wrap,
+            );
             if line_i == cursor.line {
                 let layout_cursor = self.layout_cursor(&cursor);
                 layout_i += layout_cursor.layout as i32;