Support remote image by OSC 5379 show_picture sequence.
- Adjust the image size to the terminal cell size. - If the image size is specified in html source, skip to load the image.
* terms.c: Change time to wait for the response of "\x1b[14t\x1b[18t" from 0.1 sec to 0.5 sec.

* image.c:
  - clearImage() works.
  - Use cached image files created by w3m in getImage().

* file.c: Hack for alignment.
* file.c: nw and ni are rounded up instead of rounded off to show every corner of images.
* terms.c: Clear fd_set by FD_ZERO() before select().
* display.c: Draw underline on anchor text which is not overlapped with any image.
Read width and height from jpeg, png and gif files directly instead of executing w3mimgdisplay -size.
Determine the format of an image file by its header data not by its file name suffix.
Minor fixes of parseImageHeader().
Don't download image files whose size is specified in <img> tag.
Add -sixel option which supports image processing by img2sixel.
Init pixel_per_{char|line}_i if get_pixel_per_cell() fails.
Cache image files if at all possible and convert them to sixel when -sixel option is specified.
Minor fix.
Remove close_tty() from setup_child() because close_tty() sometimes interrupts loadGeneralFile() in loadImage() and corrupt image data can be cached in ~/.w3m.
Draw underline on anchor which contains cboth text and images.
system() -> fork()&execvp()
Show the first frame of animation gif files.
Add declaration of get_pixel_per_cell().
img2sixel exits by Ctrl+C. Enable GIF Animation if 'I' is pressed to show it.