Fix graphical glitch wide chars would leave behind

Wide chars now 'own' the character next to them and are entirely responsible
for drawing and clean up

src/renderer/BaseRenderLayer.ts

@@ -128,6 +128,11 @@ export abstract class BaseRenderLayer implements IRenderLayer {
   }
 
   protected drawChar(terminal: ITerminal, char: string, code: number, width: number, x: number, y: number, fg: number, underline: boolean = false): void {
+    // Clear the cell next to this character if it's wide
+    if (width === 2) {
+      this.clearCells(x + 1, y, 1, 1);
+    }
+
     let colorIndex = 0;
     if (fg < 256) {
       colorIndex = fg + 1;

src/renderer/ForegroundRenderLayer.ts

@@ -48,6 +48,14 @@ export class ForegroundRenderLayer extends BaseRenderLayer {
         const code: number = <number>charData[CHAR_DATA_CODE_INDEX];
         const char: string = charData[CHAR_DATA_CHAR_INDEX];
         const attr: number = charData[CHAR_DATA_ATTR_INDEX];
+        const width: number = charData[CHAR_DATA_WIDTH_INDEX];
+
+        // The character to the left is a wide character, drawing is owned by
+        // the char at x-1
+        if (width === 0) {
+          this._state.cache[x][y] = null;
+          continue;
+        }
 
         // Skip rendering if the character is identical
         const state = this._state.cache[x][y];
@@ -59,7 +67,7 @@ export class ForegroundRenderLayer extends BaseRenderLayer {
 
         // Clear the old character if present
         if (state && state[CHAR_DATA_CODE_INDEX] !== 32 /*' '*/) {
-          this.clearCells(x, y, 1, 1);
+          this.clearChar(x, y);
         }
         this._state.cache[x][y] = charData;
 
@@ -102,11 +110,20 @@ export class ForegroundRenderLayer extends BaseRenderLayer {
           this.drawBottomLineAtCell(x, y);
         }
 
-        const width: number = charData[CHAR_DATA_WIDTH_INDEX];
         this.drawChar(terminal, char, code, width, x, y, fg);
 
         this._ctx.restore();
       }
     }
   }
+
+  private clearChar(x: number, y: number): void {
+    let colsToClear = 1;
+    // Clear the adjacent character if it was wide
+    const state = this._state.cache[x][y];
+    if (state && state[CHAR_DATA_WIDTH_INDEX] === 2) {
+      colsToClear = 2;
+    }
+    this.clearCells(x, y, colsToClear, 1);
+  }
 }