Optimize contexts for reading frequently

This was a recommendation from Edge.
Docs here: https://html.spec.whatwg.org/multipage/canvas.html#concept-canvas-will-read-frequently

addons/xterm-addon-webgl/src/atlas/WebglCharAtlas.ts

@@ -110,7 +110,10 @@ export class WebglCharAtlas implements IDisposable {
     this._tmpCanvas = document.createElement('canvas');
     this._tmpCanvas.width = this._config.scaledCellWidth * 4 + TMP_CANVAS_GLYPH_PADDING * 2;
     this._tmpCanvas.height = this._config.scaledCellHeight + TMP_CANVAS_GLYPH_PADDING * 2;
-    this._tmpCtx = throwIfFalsy(this._tmpCanvas.getContext('2d', { alpha: this._config.allowTransparency }));
+    this._tmpCtx = throwIfFalsy(this._tmpCanvas.getContext('2d', {
+      alpha: this._config.allowTransparency,
+      willReadFrequently: true
+    }));
   }
 
   public dispose(): void {

src/browser/ColorManager.ts

@@ -89,7 +89,9 @@ export class ColorManager implements IColorManager {
     const canvas = document.createElement('canvas');
     canvas.width = 1;
     canvas.height = 1;
-    const ctx = canvas.getContext('2d');
+    const ctx = canvas.getContext('2d', {
+      willReadFrequently: true
+    });
     if (!ctx) {
       throw new Error('Could not get rendering context');
     }