alpha_composite should be like paste and allow negative destinations

[CarlKCarlK]

If you try to use alpha_composite with a negative dest, it produces an error message. For example:

from PIL import Image
solid_green = Image.new("RGBA",(100,100),(0,255,0,255))
semidark = Image.new("RGBA",(100,100),(0,0,0,127))

ac_result = solid_green.copy()
ac_result.alpha_composite(semidark,(-50,50))
ac_result.show()
# => ValueError: Destination must be non-negative

Contrast this with paste, which allows negative destinations:

from PIL import Image
solid_green = Image.new("RGBA",(100,100),(0,255,0,255))
semidark = Image.new("RGBA",(100,100),(0,0,0,127))

paste_result = solid_green.copy()
paste_result.paste(semidark,(-50,50),semidark)
paste_result.show() # Works but colors aren't alpha composited

Negatives do indicate clipping on the left or top but note that alpha_composite (like paste) already allows clipping on the right and bottom.

from PIL import Image
solid_green = Image.new("RGBA",(100,100),(0,255,0,255))
semidark = Image.new("RGBA",(100,100),(0,0,0,127))

ac_result = solid_green.copy()
ac_result.alpha_composite(semidark,(50,50))
ac_result.show() # Works great!

Until this issue is addressed, here is a work around:

def alpha_composite_workaround(image0, image1, dest):
    x1,y1 = dest
    if x1 >= 0 and y1 >= 0:
        image0.alpha_composite(image1, dest=(x1, y1))
    else:
        temp = Image.new("RGBA", image0.size, (0, 0, 0, 0))
        temp.paste(image1, (x1, y1))
        image0.alpha_composite(temp)

from PIL import Image
solid_green = Image.new("RGBA",(100,100),(0,255,0,255))
semidark = Image.new("RGBA",(100,100),(0,0,0,127))

ac_result = solid_green.copy()
alpha_composite_workaround(ac_result,semidark,(-50,50))
ac_result.show()

[radarhere]

alpha_composite was added in #2595 by @wiredfool. Did he have any thoughts on this?

[radarhere]

I've created #5313 to resolve this.