For sixel, no need to round image size to multiple of character size.

[rokuyama]

With this fix combined with fix to libsixel:

https://github.com/saitoha/libsixel/pull/170

browsing on slow machines (e.g., when floating-point calculation is emulated by kernel) gets significantly speed up.

Note that if i->cache->a_width (i->cache->a_height) is zero, width (height) should be set to zero. Otherwise, image in screen boundary (i.e., partially displayed) becomes strangely resized.