Specify highp sampler2D for clipping plane textures to improve reliability on some devices

[likangning93]

Fixes #9023.

Some devices with Mali GPUs (mostly Android, but also seen on some Chromebooks) were having trouble with clipping plane reliability, apparently due to precision problems. As observed in #8311, WebGL implementations are free to use lower precision sampling if the precision isn't specified, so this PR explicitly specifies highp on texture sampling for clipping planes. This seems to help on the devices in the issue above.

I also tested Model, pnts, b3dm, i3dm, and Globe clipping planes with these changes on Chrome and Firefox in Linux and on Internet Explorer and Chrome in Windows, both using float textures where supported and using Cesium.ClippingPlaneCollection.useFloatTexture = function() { return false; }; to force the uint8 RGBA codepath, and it looks like this doesn't cause problems there.

@lilleyse can you test in Safari on macos?

[cesium-concierge]

Thanks for the pull request @likangning93!

• ✔️ 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.

[lilleyse]

Works in Safari on my 2015 Macbook with AMD card.

I tested Cesium.ClippingPlaneCollection.useFloatTexture = function() { return false; }; as well.

[lilleyse]

Thanks @likangning93!