Remove unnecessary ensure_unpack calls

[pierotofy]

Closes #253

I've tested this by running:

import rawpy

with rawpy.imread("iss030e122639.NEF") as f:
	print(f.sizes)
	print(f.num_colors)
	print(f.color_desc)
	print(f.raw_colors_visible)

And verifying that outputs match between this branch and main.

Now, raw_colors_visible accesses raw_colors, which still calls ensure_unpack. To get a better benchmark, one can do:

import rawpy

with rawpy.imread("iss030e122639.NEF") as f:
	print(f.sizes)
	print(f.num_colors)
	print(f.color_desc)

[pt:~/Documents/rawpy/test] [venv] unpackopt+* ± hyperfine 'python test.py'
Benchmark 1: python test.py
  Time (mean ± σ):     106.9 ms ±  19.7 ms    [User: 521.1 ms, System: 144.5 ms]
  Range (min … max):    92.2 ms … 175.6 ms    16 runs

vs. main branch:

[pt:~/Documents/rawpy/test] [venv] main+* 4s ± hyperfine 'python test.py'
Benchmark 1: python test.py
  Time (mean ± σ):     345.6 ms ±  12.3 ms    [User: 1732.8 ms, System: 491.4 ms]
  Range (min … max):   319.7 ms … 359.2 ms    10 runs

[letmaik]

Thanks for the PR!