bug#12463: 24.2; pos-visible-in-window-p gets slower over time

[Chong Yidong]

Eli Zaretskii <eliz@gnu.org> writes:

>> ***************
>> *** 8884,8892 ****
>>     DEFSYM (Qxbm, "xbm");
>>     ADD_IMAGE_TYPE (Qxbm);
>>   
>> -   define_image_type (&xbm_type, 1);
>> -   define_image_type (&pbm_type, 1);
>
> Why are these two lines being removed, while the corresponding
> ADD_IMAGE_TYPE lines are not?

This change moves define_image_type into lookup_image_type, so that
these two image types are handled just like the other image types, for
simplicity.  The ADD_IMAGE_TYPE stuff is also done for the non xmb/pbm
image types.