Add check for 16-bit highp value

[anandthakker]

Since #8183 (and the followup in #8496), we are packing pairs of colors into a single highp vec4. This strategy means that we have a hard requirement for highp providing at least 16 bits of precision.

It's possible that this color bug could be caused by a device that doesn't provide 16-bit highps.

We can add a check at startup, using glGetShaderPrecisionFormat, so that if a device doesn't have enough precision, we fail with an error message instead of trying to proceed.

[tobrun]

LMK if you want to do some CI testing on this, very interested in running this against +220 devices.

[anandthakker]

Do we have any good way to expose a flag controlling whether or not this information gets logged?

Relatedly: there are a couple of unit tests that make assertions based on logging output, and adding this diagnostic logging will mess them up, e.g. here.