Accept alternate spelling for Netscape loop extension.
Fix zero-width/height images by defaulting to screen_width/height.

This is what Firefox, Chrome do.
Revert "GIF library: Declare image dimensions as unsigneds."

This reverts commit 47be9f9ac6f2ca438440773d3b4d6a385277b081.
Testie: Report subtests on script failure.
Fix integer overflow by casting to unsigned as appropriate.

Recall the problem is that some variables (e.g., `uint16_t gfi->width`)
get promoted to int, and that very large GIFs might have signed 32-bit
sizes. The previous fix (just set gfi->width/height to unsigned) led
to lots of signedness warnings. We want to avoid the warnings. Really
what should be done is change all position variables to unsigned.
But I tried that, and it led to test failures. Scared of debugging the
failures, back off and fix the problem in a more targeted way.

Really a set of GIFs should be processed with clang's integer undefined
behavior sanitizer.

Conflicts:
	src/gifread.c
	src/quantize.c
Add zero-width image test.