In the image_remove function, slideshow_change_image needs to be called before
removing the image from the filelist.  Because of this, the "x of y" displayed
by --draw-filename is wrong (y is 1 too high).

This commit introduces a new 'render' argument to slideshow_chingae_image.
The image_remove function calls it with render=0, edits the filelist and then
runs by winwidget_render_image itself.

Patch by Yu-Jie Lin.

Fixes the annoying 100% zoom and then continuing where it was.

The ButtonRelease check only knew pan mode, so releasing the blur key changed
to the next image.

Does not yet work with initial zoom value != 100%