Move webgl-specific shared code into addon

Never got moved when canvas addon was removed

src/browser/renderer/shared/Constants.ts

@@ -3,12 +3,4 @@
  * @license MIT
  */
 
-import { isFirefox, isLegacyEdge } from 'common/Platform';
-
 export const INVERTED_DEFAULT_COLOR = 257;
-
-export const DIM_OPACITY = 0.5;
-// The text baseline is set conditionally by browser. Using 'ideographic' for Firefox or Legacy Edge
-// would result in truncated text (Issue 3353). Using 'bottom' for Chrome would result in slightly
-// unaligned Powerline fonts (PR 3356#issuecomment-850928179).
-export const TEXT_BASELINE: CanvasTextBaseline = isFirefox || isLegacyEdge ? 'bottom' : 'ideographic';