ICO: Only save relevant sizes

[hugovk]

Fixes #2266.

Includes PR #2265 that fixes #2264.

See the antepenultimate commit for a failing case for this, and the last two commits to fix it.

The return from filter was being ignored.

For width, height = 64, 64 it returns:
[(16, 16), (24, 24), (32, 32), (48, 48), (64, 64)]

For width, height = 256, 256 it returns:
[(16, 16), (24, 24), (32, 32), (48, 48), (64, 64), (128, 128), (256, 256)]

---

Running the test in #2266 gives:

set([(16, 16), (48, 48), (32, 32)])
set([(16, 16), (24, 24), (48, 48), (32, 32)])

$ identify python.ico
python.ico[0] ICO 16x16 16x16+0+0 32-bit sRGB 15.1KB 0.000u 0:00.002
python.ico[1] ICO 32x32 32x32+0+0 32-bit sRGB 15.1KB 0.016u 0:00.007
python.ico[2] ICO 48x48 48x48+0+0 32-bit sRGB 15.1KB 0.016u 0:00.013
$ identify saved.ico
saved.ico[0] PNG 16x16 16x16+0+0 8-bit sRGB 6.66KB 0.000u 0:00.000
saved.ico[1] PNG 24x24 24x24+0+0 8-bit sRGB 6.66KB 0.000u 0:00.004
saved.ico[2] PNG 32x32 32x32+0+0 8-bit sRGB 6.66KB 0.000u 0:00.007
saved.ico[3] PNG 48x48 48x48+0+0 8-bit sRGB 6.66KB 0.000u 0:00.010