This is needed to have code for creating an in-memory sRGB profile using
the (floating-ppoint) numbers from the sRGB spec and having the
fixed-point values in the profile match what they are in other software
(such as GIMP).
It has the side effect of making the FixedPoint ctor no longer constexpr
(which seems fine; nothing was currently relying on that).
Some of FixedPoint's member functions don't round yet, which requires
tweaking a test.
We weren't properly iterating the extension blocks and thought we
encountered an unexpected extension map block, when we really should
have just skipped over it.
This library can be used (for the most part) by kernel drivers as well
as user mode. For this reason FixedPoint is used rather than floating
point, but kept to a minimum.