Set SMaskInData to 1 for LA PDFs

[radarhere]

The combination of #7299 and #7316 has led to a lint error in main.

This fixes that, while also applying the RGBA changes from #7316 to the new LA saving from #7299.

• ColorSpace is removed for LA, since the specification states under 7.4.9 "JPXDecode Filter" that "ColorSpace shall be optional since JPEG2000 data contain colour space specifications"
• SMaskInData is set to 1, allowing

from PIL import Image
im = Image.open("/Users/andrewmurray/pillow/Pillow/Tests/images/transparent.png")
im.convert("LA").save("la.pdf")

followed by an ImageMagick convert la.pdf la.png to result in an image with transparency.