Reword "gamma" and polish image description events

wp_image_description_v1 events are tuned to match their factory counter-parts, and any differences are explicitly commented.

The rules on when these events are sent are also improved. Now all events that could be sent also must be sent - not by theory but by what information the compositor has at hand anyway. Therefore this does not require compositors to e.g. extract primaries from ICC files, but if they were to do it anyway and have correctly meaningful results, they can also send them in addition to the ICC file.

This means that clients may get the same information in multiple different events. The intention is to allow clients to choose the form of information that best suits them, e.g. CICP codes vs. primaries and white point CIE coordinates. The compositor does not know and should not care which form is the best for the client. The simplest solution is to just send them all.

"Gamma" is renamed "power function" and "exponent", just to be more explicit and avoid the overloaded term "gamma".

Behavior of destruction is more explicitly defined.