clip size can override user-requested size for fullsize screenshot

[j-h-a]

Description
Output image is too big when user-requested size is smaller than internally grabbed image size.

To reproduce

webkit2png -W 800 -H 480 http://localhost:8080 -F

Expected behaviour
Output image should be 800x480

Actual behaviour
Output image is 800x600.

Proposed Solution
If user-requested fullsize grab is smaller than the internally grabbed size (if grabbed size is expanded to ensure the clipped thumb has enough content) then the final output for the fullsize image should be cropped to the user-requested size.

Workaround
Until fixed, users can add --scale=1 (in cases where clipped thumb is not wanted).