fix issue #685 [tx] subroutinizer 64K limit

[blueshade7]

use "unsigned" in place of "unsigned short" as type of offsets and lengths
verified the fix using the attached CFF2 font uni7e7c.otf

[cjchapman]

@blueshade7 I'm OK with the change as it is now, so I'll approve, but I'd like it even better if you used uint32_t and int32_t instead of unsigned and int in order to make the bits needed explicit. I'm trying to make similar changes throughout the code base when I work on size related issues like this one. Thanks for considering this request.

[readroberts]

I'll second Chris. Going forward, when you need to edit any code, please change to uint32_t and int32_t types. I will try and remember this as well. Otherwise, this is a simple change and looks good. I'm marking it approved so that you cam squash and merge when you are done.

[blueshade7]

OK I re-worked it to use explicit sized types.

[cjchapman]

@blueshade7 Thanks!