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;