Use plugin method directly when saving PDFs

[radarhere]

Resolves #9545

from PIL import Image
im = Image.new("RGB", (1, 1))
im.save("out.pdf")

currently raises

Traceback (most recent call last):
  File "demo.py", line 3, in <module>
    im.save("out.pdf")
  File "PIL/Image.py", line 2713, in save
    save_handler(self, fp, filename)
  File "PIL/PdfImagePlugin.py", line 262, in _save
    image_ref, procset = _write_image(im, filename, existing_pdf, image_refs)
  File "PIL/PdfImagePlugin.py", line 151, in _write_image
    Image.SAVE["JPEG"](im, op, filename)
KeyError: 'JPEG'

The problem is that at line 151 (and for mode RGBA at line 154) JpegImagePlugin (and Jpeg2KImagePlugin) haven't necessarily been imported.

Pillow/src/PIL/PdfImagePlugin.py

Lines 150 to 154 in c722aae

	elif decode_filter == "DCTDecode":
	Image.SAVE["JPEG"](im, op, filename)
	elif decode_filter == "JPXDecode":
	del dict_obj["BitsPerComponent"]
	Image.SAVE["JPEG2000"](im, op, filename)

One solution would be to import the plugins, so that register_save() is called, and still use Image.SAVE. However, lint raises an error that the imported plugins are not used.

I think it is better to actually call the plugin method directly, like MpoImagePlugin does.

from . import JpegImagePlugin

JpegImagePlugin._save(im, op, filename)