Add code to IGlyphIdentifer, use it

It turns out that `glyph.char.charCodeAt(0)` isn't equivalent to the
character's actual code because of utf-16 garbage, so this adds
`glyph.code` to the `glyph` object so that we can use the `code` value,
and don't need to regenerate it.

src/renderer/BaseRenderLayer.ts

@@ -240,7 +240,7 @@ export abstract class BaseRenderLayer implements IRenderLayer {
   protected drawChar(terminal: ITerminal, char: string, code: number, width: number, x: number, y: number, fg: number, bg: number, bold: boolean, dim: boolean): void {
     const atlasDidDraw = this._charAtlas && this._charAtlas.draw(
       this._ctx,
-      {char, bg, fg, bold: bold && terminal.options.enableBold, dim},
+      {char, code, bg, fg, bold: bold && terminal.options.enableBold, dim},
       x * this._scaledCellWidth + this._scaledCharLeft,
       y * this._scaledCellHeight + this._scaledCharTop
     );

src/renderer/atlas/DynamicCharAtlas.ts

@@ -121,7 +121,7 @@ export default class DynamicCharAtlas extends BaseCharAtlas {
     // to draw overlapping glyphs from the atlas:
     // https://github.com/servo/webrender/issues/464#issuecomment-255632875
     // https://webglfundamentals.org/webgl/lessons/webgl-text-texture.html
-    return glyph.char.charCodeAt(0) < 256;
+    return glyph.code < 256;
   }
 
   private _toCoordinates(index: number): [number, number] {

src/renderer/atlas/StaticCharAtlas.ts

@@ -34,7 +34,7 @@ export default class StaticCharAtlas extends BaseCharAtlas {
   }
 
   private _isCached(glyph: IGlyphIdentifier, colorIndex: number): boolean {
-    const isAscii = glyph.char.charCodeAt(0) < 256;
+    const isAscii = glyph.code < 256;
     // A color is basic if it is one of the standard normal or bold weight
     // colors of the characters held in the char atlas. Note that this excludes
     // the normal weight _light_ color characters.
@@ -86,7 +86,7 @@ export default class StaticCharAtlas extends BaseCharAtlas {
 
     ctx.drawImage(
       this._texture,
-      glyph.char.charCodeAt(0) * charAtlasCellWidth,
+      glyph.code * charAtlasCellWidth,
       colorIndex * charAtlasCellHeight,
       charAtlasCellWidth,
       this._config.scaledCharHeight,

src/renderer/atlas/Types.ts

@@ -8,6 +8,7 @@ export const DIM_OPACITY = 0.5;
 
 export interface IGlyphIdentifier {
   char: string;
+  code: number;
   bg: number;
   fg: number;
   bold: boolean;