Render the cursor in the WebGL canvas

[xzfc]

#4082

The RectangleRenderer is updated not just to draw cell backgrounds, but the cursor as well (bar and underscore cursors are 1px wide rectangles, blur cursor consists of four 1px rectangles). The block cursor is implemented by changing cell attributes.

The CursorRenderLayer is dropped.

[Tyriar]

I notice the cursor fg is unexpectedly blue sometimes, I would have thought this is black:

[Tyriar]

Underline and bar cursors don't seem to update correctly atm

[Tyriar]

Changing to light theme in demo still shows white block cursor:

[Tyriar]

Fixed the blue problem, was related to rgba format being used as rgb f64cb15

[Tyriar]

The light theme issue is actually a problem outside of this PR as the cursor colors don't get set in the theme so the defaults are used.

[Tyriar]

@xzfc great work on this! I made a few fixes and I think this is good to go

[Tyriar]

Just need to fix the tests:

  1) WebGL Renderer Integration Tests
       dispose removes renderer canvases:

      AssertionError: expected 2 to equal 3
      + expected - actual

      -2
      +3
      
      at Context.<anonymous> (addons\xterm-addon-webgl\test\WebglRenderer.api.ts:28:12)
      at Generator.next (<anonymous>)
      at fulfilled (addons\xterm-addon-webgl\out-test\WebglRenderer.api.js:5:58)