Clean up char atlas constant imports/exports

- Ensures that everything is only defined in one place, and everything
  imports from that place.
- Moves CHAR_ATLAS_CELL_SPACING into a subdirectory of shared/ so that
  CharAtlasGenerator can pull from it.

This addresses the comments on
https://github.com/xtermjs/xterm.js/pull/1307/files/454fd4bfb1ab5c9ece

src/renderer/BaseRenderLayer.ts

@@ -5,12 +5,11 @@
 
 import { IRenderLayer, IColorSet, IRenderDimensions } from './Types';
 import { CharData, ITerminal, ITerminalOptions } from '../Types';
-import { acquireCharAtlas, CHAR_ATLAS_CELL_SPACING } from './CharAtlas';
+import { DIM_OPACITY, INVERTED_DEFAULT_COLOR } from './atlas/Types';
+import { CHAR_ATLAS_CELL_SPACING } from '../shared/atlas/Types';
+import { acquireCharAtlas } from './CharAtlas';
 import { CHAR_DATA_WIDTH_INDEX, CHAR_DATA_CHAR_INDEX } from '../Buffer';
 
-export const INVERTED_DEFAULT_COLOR = -1;
-const DIM_OPACITY = 0.5;
-
 export abstract class BaseRenderLayer implements IRenderLayer {
   private _canvas: HTMLCanvasElement;
   protected _ctx: CanvasRenderingContext2D;

src/renderer/CharAtlas.ts

@@ -10,8 +10,6 @@ import { isFirefox } from '../shared/utils/Browser';
 import { generateCharAtlas, ICharAtlasRequest } from '../shared/CharAtlasGenerator';
 import { generateConfig, configEquals } from './atlas/CharAtlasUtils';
 
-export const CHAR_ATLAS_CELL_SPACING = 1;
-
 interface ICharAtlasCacheEntry {
   bitmap: HTMLCanvasElement | Promise<ImageBitmap>;
   config: ICharAtlasConfig;

src/renderer/LinkRenderLayer.ts

@@ -7,7 +7,8 @@ import { ILinkHoverEvent, ITerminal, ILinkifierAccessor, IBuffer, ICharMeasure,
 import { CHAR_DATA_ATTR_INDEX } from '../Buffer';
 import { GridCache } from './GridCache';
 import { FLAGS, IColorSet, IRenderDimensions } from './Types';
-import { BaseRenderLayer, INVERTED_DEFAULT_COLOR } from './BaseRenderLayer';
+import { INVERTED_DEFAULT_COLOR } from './atlas/Types';
+import { BaseRenderLayer } from './BaseRenderLayer';
 
 export class LinkRenderLayer extends BaseRenderLayer {
   private _state: ILinkHoverEvent = null;

src/renderer/TextRenderLayer.ts

@@ -6,8 +6,9 @@
 import { CHAR_DATA_ATTR_INDEX, CHAR_DATA_CODE_INDEX, CHAR_DATA_CHAR_INDEX, CHAR_DATA_WIDTH_INDEX } from '../Buffer';
 import { FLAGS, IColorSet, IRenderDimensions } from './Types';
 import { CharData, IBuffer, ICharMeasure, ITerminal } from '../Types';
+import { INVERTED_DEFAULT_COLOR } from './atlas/Types';
 import { GridCache } from './GridCache';
-import { BaseRenderLayer, INVERTED_DEFAULT_COLOR } from './BaseRenderLayer';
+import { BaseRenderLayer } from './BaseRenderLayer';
 
 /**
  * This CharData looks like a null character, which will forc a clear and render

src/renderer/atlas/Types.ts

@@ -6,7 +6,6 @@
 import { FontWeight } from 'xterm';
 import { IColorSet } from '../Types';
 
-export const CHAR_ATLAS_CELL_SPACING = 1;
 export const INVERTED_DEFAULT_COLOR = -1;
 export const DIM_OPACITY = 0.5;
 

src/shared/CharAtlasGenerator.ts

@@ -4,6 +4,7 @@
  */
 
 import { FontWeight } from 'xterm';
+import { CHAR_ATLAS_CELL_SPACING } from './atlas/Types';
 import { isFirefox } from './utils/Browser';
 
 declare const Promise: any;
@@ -29,8 +30,6 @@ export interface ICharAtlasRequest {
   allowTransparency: boolean;
 }
 
-export const CHAR_ATLAS_CELL_SPACING = 1;
-
 /**
  * Generates a char atlas.
  * @param context The window or worker context.

src/shared/atlas/Types.ts

@@ -0,0 +1 @@
+export const CHAR_ATLAS_CELL_SPACING = 1;