Remove ACCURATE_GAMMA_ADJUSTMENT, since it makes the implementation
uglier, isn't guaranteed to make gamma adjustment accurate and is much
slower. gamma_accurate_blend4 should either be always used or not
exist based on compilation flags, so there is no need to have it in
its own function. Finally, we should use AK/SIMD.h instead of defining
our own f32x4 type.
This is a hack which can be removed once GitHub Actions changes the
default version to clang 11.
This is apparently sometime in mid-December.
Note, clang-11 is not currently available on Ubuntu 20.04. However,
GitHub Actions uses 20.04, which probably means clang-11 will
become available around that time for all 20.04 users.
Switch over to gamma-aware interpolation. This causes color gradients
to not look so dark in the middle. SIMD optimized code is provided for
sse1 enabled builds.
Fixes#1342.
