WebGLCapabilities: Use mediump by default

[mrdoob]

Description

This PR is just to see what would break if we used mediump by default and also to test if there are any performance improvements.

[Mugen87]

https://raw.githack.com/mrdoob/three.js/mediump/examples/index.html

[marcofugaro]

Hmm, wonder what's the discussion that led to this test

[Mugen87]

Some context: #14570

[mrdoob]

On Pixel 4a the lighting is incorrect but the performance improvement is considerable...

https://threejs.org/examples/webgl_materials_variations_physical.html (8fps)
https://raw.githack.com/mrdoob/three.js/mediump/examples/webgl_materials_variations_physical.html (14fps)

https://threejs.org/examples/webgl_materials_physical_clearcoat.html (25fps)
https://raw.githack.com/mrdoob/three.js/mediump/examples/webgl_materials_physical_clearcoat.html (42fps)

[Mugen87]

There seems to be artifacts with certain demos (tested with Pixel 4a). E.g.

https://raw.githack.com/mrdoob/three.js/mediump/examples/webgl_lights_rectarealight.html (34fps)

https://threejs.org/examples/webgl_lights_rectarealight.html (18fps)

Probably because of #14570 (comment).

[Mugen87]

There are missing hot spots with the marching cubes example.

https://raw.githack.com/mrdoob/three.js/mediump/examples/webgl_marchingcubes.html

https://threejs.org/examples/webgl_marchingcubes