When building the GPUBuffer during the prepare pass, we want to be
able to refer to the UV rect (and other information) about images
and render tasks. However, we don't know this information until
after the resource cache block that occurs after prepare (waiting
for blob / glyph worker threads to complete).
Previously, we would defer these queries until batching and then
reference these via a GPU cache address in the instances, which
is complicated and inefficient (making instances larger than they
need to be).
With this change, GPUBuffer supports recording "patch" locations in
the buffer that UV rects should be added. After the render task
graph is built, the GPUBuffer can resolve and patch those locations,
meaning that batching is much simpler (and reducing the number of
indirect fetches required in vertex shaders).
Differential Revision: https://phabricator.services.mozilla.com/D159236
7b751b5e60