Remove older details

[ccameron-chromium]

This spec is several years old and has accumulated some cruft. Clear some of it out. Also, remove extra background information and definitions. This information is helpful, but is currently cluttering the spec, and we are trying to pin down the behavior changes.

In particular:

• From "Current Limitations", remove remarks about perceptual-linear unorm8 encoding, because the stored encoding of 8-bit contents is unchanged by this specification (unorm8-srgb affects only blending and fragment math space).
• Remove "Current Usage and Workarounds" section. Most of the behavior documented in this section is obsolete.
• Shorten "Requests for this Feature" section to not specify particular vendor applications.
• Remove the "Background" section and replace with a "Related Specifications" section. Do not define the color spaces in this document, rather, link to the CSS Color Level 4 specification.
• Move the "Selecting the best color space match for the user agent's display device" to an "Examples" section at the end. Remove speculation about a TBD isHDR method.
• Remove discussions of "srgb" and "rec-2020" color spaces.
• Separate subsections for "2D canvas behavior" versus "WebGL behavior", since the effects of the spec are different on the two.
• Move discussion of srgb-encoded pixel values to WebGL behavior section.
• Clarify that undefined or unsupported color spaces should default to srgb with unorm8.
• Remove the advice that "Implementations should not expose color spaces that are unreasonable for the display".

[ccameron-chromium]

Thanks all! Any further requests?