Extended brightness range rendering

The current spec does not clearly indicate what should be done with color values that land outside of the standard dynamic range gamut of the screen (colors that, when converted to the color space of the screen, land outside of the `[0, 1]` interval).

The following proposal indicates a default behavior (that matches existing implementations and WebGPU's default behavior), and provides a mechanism for opting in to extended-range brightness.
https://github.com/ccameron-chromium/webgl-hdr/blob/master/EXPLAINER.md

Start by reading the linked EXPLAINER.md, then compare its proposal with the current WebGL specification. The work is complete when the specification clearly defines handling of out-of-range color values and the proposed opt-in mechanism for extended-range brightness.

Written by the indexing model from the issue text.