merge mesa 21.3.7.

Merge mesa 19.1.7

Use a name for the largest representable float

Actually intended to appease vax which has a floating point format
that doesn't match the same range as IEEE754 float.

(why not exp2f?)

from maya:

Import mesa 18.3.4.

Mesa 18.3.4 implements the OpenGL 4.5 API.
Some drivers don't support all the features required in OpenGL 4.5.