Fix blending states

[lilleyse]

The blending equation for the alpha channel was incorrect in master. It would end up doing
srcAlpha * srcAlpha + dstAlpha * (1.0 - srcAlpha)

After the fix the equation is srcAlpha * 1.0 + dstAlpha * (1.0 - srcAlpha)

Also fixed ADDITIVE_BLEND which had the same problem.

Noticed while working on #4381. I guess it never came up before because we never alpha blend a framebuffer onto another framebuffer.

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.
• ❔ 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.

[IanLilleyT]

Looks good @lilleyse