Do not retry specified formats if they failed when opening

[radarhere]

When opening an image, Pillow loop through the different possible formats.

Pillow/src/PIL/Image.py

Lines 3242 to 3244 in 43bb035

	def _open_core(fp, filename, prefix, formats):
	for i in formats:
	i = i.upper()

If I add a print statement to see which formats it checks,

    def _open_core(fp, filename, prefix, formats):
        for i in formats:
            print(i)
            i = i.upper()

and then attempt to open a SPIDER image, while only asking Pillow to check the JPEG format,

from PIL import Image
im = Image.open("Tests/images/hopper.spider", formats=["JPEG"])

it prints

JPEG
JPEG
Traceback (most recent call last):
  File "Tests/test_image.py", line 2, in <module>
    im = Image.open("Tests/images/hopper.spider", formats=["JPEG"])
  File "PIL/Image.py", line 3284, in open
    raise UnidentifiedImageError(msg)
PIL.UnidentifiedImageError: cannot identify image file 'Tests/images/hopper.spider'

JPEG is checked twice.

This is because in the following code, Pillow doesn't recognise that formats is a fixed list, and that calling init() will not increase the number of formats to try.

Pillow/src/PIL/Image.py

Lines 3268 to 3272 in 43bb035

	im = _open_core(fp, filename, prefix, formats)
	if im is None:
	if init():
	im = _open_core(fp, filename, prefix, formats)

So this PR changes it to only retry _open_core if formats has not been specified.

    im = _open_core(fp, filename, prefix, formats)

    if im is None and formats is ID:
        if init():
            im = _open_core(fp, filename, prefix, formats)

But wait, you say, you've also skipped calling init(). What if the needed format hasn't been imported before the call to _open_core?

Not to worry. _open_core already automatically calls init() if there is something surprising in formats

Pillow/src/PIL/Image.py

Lines 3242 to 3246 in 43bb035

	def _open_core(fp, filename, prefix, formats):
	for i in formats:
	i = i.upper()
	if i not in OPEN:
	init()