Fix a bug with UV rects for blob images in the GPU cache.
Fix a bug with UV rects for blob images in the GPU cache.

Previously, the texture cache ID for blob images would only
get added to the requested items hash set when the blob
was new or out of date. This change ensures that blob images
get the associated texture cache ID added to the requested
items hash set unconditionally.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender