Speed up and simplify GIF optimization

[akx]

While working on #10072, I was looking for uses of ImageMath around Pillow, and this seemed like a pretty complex construction and there indeed was a simpler one lurking in there... 😄

ImageChops was optimized in 4b5a6e3 and Image.point would be optimized by #9743. #9829 would improve (though currently slightly slow down 😞) quantization accuracy.

There are no new tests or benchmarks committed; the test added in #7568 covers this.

This also requires less intermediate images, so peak memory use decreases too (30% to 80% on my machine, depending on whether we're talking shared palette or a per-frame one).

I benchmarked this locally:

-------------------------- benchmark 'gif': 4 tests, 2 sources ---------------------------
Name (time in ms)                             0001_5600015 Min  0002_b491b6e Min      ΔMin
------------------------------------------------------------------------------------------
test_save_gif[shared-palette-plain]                    13.4083           12.9534     -3.4%
test_save_gif[shared-palette-optimize]                 82.2019           13.9369    -83.0%
test_save_gif[per-frame-palette-plain]                443.2780          429.3091     -3.2%
test_save_gif[per-frame-palette-optimize]             462.2850          439.6624     -4.9%
------------------------------------------------------------------------------------------

The per-frame-palette case doesn't get optimized much since it's dominated by palette quantization, not this.