Kitty inline image & base64 encoding fixes

[bptato]

• base64_encode used to return a char buffer, I replaced this with a Str struct. This also means that Strcat_char does pointless bounds checking in this function which could be done once... no noticeable performance impact for me but if deemed necessary we could also write directly to the char pointer. Or use a gc_malloc'ed buffer as before, then call Strnew_charp.
• I fixed an oversight of mine where base64 padding could be applied incorrectly.
• Finally I changed the convert call to specify that ImageMagick should extract the first frame only from gifs. It otherwise extracted all frames for animations, which made w3m look for the wrong path.

[tats]

base64 padding could be applied incorrectly.

Confirmed and merged. Thanks for your contribution.

Strcat_char does pointless bounds checking

I've defined Strcatc and Strnulterm (like Strcat_char, but with no checks),
and replaced with them in base64_encode().