Batch background draws together to reduce draws

Adjacent cells on the same row sharing the same background color will be
drawn using the same `fillCells` call. This should make drawing an
application with a solid or mostly solid background color (e.g. vim)
faster.

I set vim to draw the background color for a file, and started scrolling
through it. Before, _drawBackground was taking around 6ms per frame.
After this commit, it was taking around 0.5ms per frame.

Based on prior experience, I'd expect these results to be more drastic
for larger terminal windows. The demo's window is pretty small.

src/renderer/TextRenderLayer.ts

@@ -128,18 +128,58 @@ export class TextRenderLayer extends BaseRenderLayer {
     }
   }
 
+  /**
+   * Draws the background for a specified range of columns. Tries to batch adjacent cells of the
+   * same color together to reduce draw calls.
+   */
   private _drawBackground(terminal: ITerminal, firstRow: number, lastRow: number): void {
+    const ctx = this._ctx;
+    const cols = terminal.cols;
+    let startX: number = 0;
+    let startY: number = 0;
+    let prevFillStyle: string | null = null;
+
+    ctx.save();
+
     this._forEachCell(terminal, firstRow, lastRow, (code, char, width, x, y, fg, bg, flags) => {
       // libvte and xterm both draw the background (but not foreground) of invisible characters,
       // so we should too.
-      const isDefaultBackground = bg >= 256;
-      if (!isDefaultBackground) {
-        this._ctx.save();
-        this._ctx.fillStyle = (bg === INVERTED_DEFAULT_COLOR ? this._colors.foreground.css : this._colors.ansi[bg].css);
-        this.fillCells(x, y, width, 1);
-        this._ctx.restore();
+      let nextFillStyle = null; // null represents default background color
+      if (bg === INVERTED_DEFAULT_COLOR) {
+        nextFillStyle = this._colors.foreground.css;
+      } else if (bg < 256) {
+        nextFillStyle = this._colors.ansi[bg].css;
       }
+
+      if (prevFillStyle === null) {
+        // This is either the first iteration, or the default background was set. Either way, we
+        // don't need to draw anything.
+        startX = x;
+        startY = y;
+      } if (y !== startY) {
+        // our row changed, draw the previous row
+        ctx.fillStyle = prevFillStyle;
+        this.fillCells(startX, startY, cols - startX, 1);
+        startX = x;
+        startY = y;
+      } else if (prevFillStyle !== nextFillStyle) {
+        // our color changed, draw the previous characters in this row
+        ctx.fillStyle = prevFillStyle;
+        this.fillCells(startX, startY, x - startX, 1);
+        startX = x;
+        startY = y;
+      }
+
+      prevFillStyle = nextFillStyle;
     });
+
+    // flush the last color we encountered
+    if (prevFillStyle !== null) {
+      ctx.fillStyle = prevFillStyle;
+      this.fillCells(startX, startY, cols - startX, 1);
+    }
+
+    ctx.restore();
   }
 
   private _drawForeground(terminal: ITerminal, firstRow: number, lastRow: number): void {