Fixes#1037
Renderers must convert the internal Kitty graphics state to a GPU
texture for rendering. For performance reasons, renderers cache the GPU
state by image ID. Unfortunately, this meant that if an image was
replaced with the same ID and was already cached, it would never be
updated on the GPU.
This PR adds the transmission time to the cache. If the transmission
time differs, we assume the image changed and replace the image.
