DomRenderer: fix touch scrolling starting from text

When scrolling with touchscreen starting from text, as soon as touch
leaves the text span, the touch event stops, making touch-scrolling able
to scroll one row only.

This fixes it by setting `pointer-events: none` to text spans, excluding
them from hit-testing so that touch events won't be constrained to them.

Fixes xtermjs#3613

src/browser/renderer/dom/DomRenderer.ts

@@ -143,6 +143,7 @@ export class DomRenderer extends Disposable implements IRenderer {
  ` display: inline-block;` + // TODO: find workaround for inline-block (creates ~20% render penalty)
  ` height: 100%;` +
  ` vertical-align: top;` +
+ ` pointer-events: none;` +
  `}`;
 
  this._dimensionsStyleElement.textContent = styles;