8105 - Explicitly enable EXT_float_blend to silence WebGL warnings

[Samulus]

Fixes #8105

You can test this PR by:

1. Opening http://localhost:8080/Apps/Sandcastle/index.html?src=Billboards.html in Firefox and observing Error: WebGL warning: drawElementsInstanced: Using format enabled by implicitly enabled extension: EXT_float_blend. For maximal portability enable it explicitly. Context.js:321:37 is printed to the console.
2. Building with this PR and verifying this message is no longer logged.

I'm using Firefox 72.0.1 on Windows 10.0.18363.418, Couldn't reproduce warning message in Chrome 79.0.3945.117

[cesium-concierge]

Thank you so much for the pull request @Samulus! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
  • Please add yourself to the contributors file!
• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

@Samulus Works perfectly. Just one small fix needed for CHANGES.md. You can put [skip ci] in the commit message or description to avoid starting another CI build.

[lilleyse]

Confirmed that this fix works in Firefox 72.0.1 and Ubuntu 18.04.2.